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Teoreties kan die uitbreiding van die heelal versnelling weens swaartekrag beïnvloed?

Teoreties kan die uitbreiding van die heelal versnelling weens swaartekrag beïnvloed?


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Ek het my afgevra of teoreties beïnvloed die uitbreiding van die heelal spoed of versnelling weens swaartekrag, in vergelyking met as die ruimte nie versnel nie.

Soos ek dit in my kop het, val iets na 'n planeet met swaartekrag G wat daarop inwerk. Dit het vir T tyd gedaal en het X afstand gegaan met Y oor om te gaan. Tans brei die heelal uit, dus die afstand tussen sy begin- en eindpunt is baie langer as dit aankom as toe dit begin het, of dit beïnvloed sy afstand afgelê op tyd T, of sy snelheid op tyd T. Vergelyk dit met 'n nie -uitbreiding van die heelal waar die afstand tussen begin en einde dieselfde bly, maak nie saak watter tydstip u kies nie.

Ek weet dat plaaslike swaartekragte en atoomkragte die uitbreidingskragte sterk oorrompel, sodat sterrestelsels, sonnestelsels, planete en waatlemoene nie net uitmekaar vlieg nie. Ek wonder net of die waarde van gravitasieversnelling of die spoed van 'n voorwerp teoreties weens kosmiese uitbreiding effens kan verander.


Op die oomblik word die uitbreiding van die ruimte slegs waargeneem op skale soos dié van sterrestelsels en die hele heelal. Ons sê dit met die fyn term wêreldwyd. Die uitbreiding van die ruimte vind ten minste vir eers nie binne sterrestelsels plaas nie.

Soos pela tereg in die opmerkings opgemerk het, is die gravitasie van sterrestelsels, sterre, ensovoorts sterk genoeg om die uitbreiding van die heelal te oorkom. Maar soos die algemene relatiwiteit ons vertel, is swaartekrag plaaslike, dit wil sê, dit raak slegs voorwerpe wat aansienlik naby is. Dus gaan die uitbreiding op groot skaal voort. (Ek is geen kundige nie, maar ek dink dat die rede waarom die uitbreiding nie in sterrestelsels buite kleiner stelsels plaasvind nie, is dat die materie en energie digter is, oplos in sterker gravitasievelde).

Daar word veronderstel dat die heelal met 'n groot skeuring kan eindig: dit beteken dat die uitbreiding uiteindelik sterrestelsels sou beïnvloed. Sterrestelsels, sterrestelsels, planete en uiteindelik atome sal uitmekaar geruk word weens die uitbreiding van die ruimte. Wanneer en as dit gebeur, sal nie net swaartekrag nie, maar selfs nie elektromagnetisme of die sterk krag, die magtige uitbreiding oorkom nie.

Maar hierdie situasie is hoogs hipoteties en miskien sal dit nie eers gebeur nie. Die antwoord is dus vir nou nee: die uitbreiding van die heelal kan nie swaartekragversnelling beïnvloed nie.


Galileo & # 039s Pendulum

Waarom brei die Heelal uit as swaartekrag 'n universeel aantreklike krag is? Moet die onderlinge aantrekkingskrag nie alles saam laat ineenstort en die kosmos eindig in 'n aaklige geknars van epiese afmetings nie, en elke lewende wese roep moedeloos uit voordat dit in die vergetelheid saamgepers word? (Moet die skrywer sy metafore inhou?)

Ja, enige twee deeltjies materie trek mekaar aan. Maar as ons 'n groot hoeveelheid materie oorweeg - al die aangeleenthede in 'n gegewe deel van die hele heelal - gaan dit nie almal in 'n swart gat stort nie, net al die lug in 'n kamer sal spontaan in een hoek versamel . Dink daaraan so: enige deeltjie te midde van 'n diffuse wolk van deeltjies sal gelyktydig in alle rigtings getrek word, dus sal die swaartekrag geneig wees om gemiddeld tot nul te wees. (As die deeltjies om een ​​of ander rede meer saamgepers is, is dit 'n ander storie! Wedersydse gravitasie kan dit in wolke, sterrestelsels, sterre of selfs swart gate bind.)

Wat dryf kosmiese uitbreiding? Die antwoord is steeds swaartekrag, maar ons moet dit effens anders dink: die manier van algemene relatiwiteit. Om die analogie van my vorige boodskap te gebruik, & # 8220The River of Spacetime & # 8220, as ruimtetyd 'n rivier is, is swaartekrag die stroom. Die tempo van die huidige vloei hang af van hoeveel energie daar in 'n streek van die rivier is: die energiedigtheid. Aangesien ons weet dat massa en energie intiem verwant is, sal enige stofdeeltjies in 'n streek bydra tot die energiedigtheid, wat die stroom beïnvloed, wat weer die beweging van die deeltjies sal beïnvloed. (En so aan.)

As u 'n redelike diffuse wolk van materiaal het (of dit nou donker materie of gewoon is) in 'n gebied van ruimtetyd, sal die stroom die deeltjies óf na mekaar toe dra. Watter rigting hang af van hoe hulle begin: sodra die proses begin, sal swaartekrag die res doen. In die geval van ons heelal het die oerknal die beweging begin, sodat die saak in die kosmos stroom sal skep wat deeltjies verder van mekaar af dra. Dit is die uitbreiding van die heelal.

Die verhaal is egter nog nie klaar nie. As u materie in 'n klein boksie plaas, het dit 'n hoë energiedigtheid. Dit impliseer 'n groot potensiaal vir uitbreiding deur teen die kante van die boks te druk. Die verhoging van die volume verminder die digtheid noodwendig, aangesien daar 'n vaste hoeveelheid materie binne is. Dit laat die energie weg. Dit is waar of die deeltjies met mekaar bots of nie: gemiddeldes oor die hele heelal is atome baie wyd verspreid. Daarom, terwyl atome in sterrestelsels, sterre en ander relatief klein stukkies van die kosmos bots en op die grootste skaal bots, kan ons gerus aanneem dat atome mekaar nooit tref nie. Donker materie is digter (gemiddeld weer) as normale materie, maar sover ons kan sien, is interaksies tussen donker materie-deeltjies uiters skaars. [ 1 ] En alhoewel ek 'n boks gebruik vir 'n gemaklike demonstrasie van hierdie beginsel (iets wat ek al voorheen gedoen het!), Is die algemene beginsel nie een nodig nie. Ruimtetyd-uitbreiding word aangedryf deur energie, nie deur druk op mure nie.

Materie, hetsy gewone of donker, word behoue ​​gebly: die vergroting van die houer se grootte verminder die digtheid van energie, en dus die potensiaal vir verdere uitbreiding. Donker energie daarenteen het konstante digtheid (in die eenvoudigste model), dus hoe groter die boks is, hoe meer donker energie is daar, en hoe groter potensiaal vir uitbreiding.

Soos u hopelik bymekaargekom het, is materie voldoende om ruimte-uitbreiding aan te dryf: geen donker energie benodig nie. (Ek het die bydrae uit die lig weggelaat, wat ook kosmiese uitbreiding kan dryf, maar dit het nie 'n belangrike rol gespeel sedert die heelal jonk was nie.) In werklikheid verskil donker energie fundamenteel as materie (gewoon of donker), wat 'n afnemende effek namate ruimtetyd uitbrei. Dit is waarom ek haat die term & # 8220donker energie & # 8221, omdat dit die term & # 8220donker saak & # 8221 oproep en die twee stowwe baie verskillende effekte het. Daar is egter beide materie en donker energie in die heelal wat ons het, en om te verstaan ​​waarom ruimtetyd uitbrei, vereis albei konsepte.

Aantekeninge

Ek is aangespoor om die vraag in die posttitel te vra en te beantwoord deur Peter Coles, wat spesifiek wou hê dat wetenskapskrywers duideliker moes wees oor die verduideliking van donker energie: die naam wat ons gee aan die stof wat kosmiese versnelling dryf. Sean Carroll het hier 'n verduideliking gegee, insluitend hoe nie om donker energie te beskryf. En ek bely: ek het die fout gemaak wat Sean beskryf, so beskou hierdie pos as 'n regstelling vir myself!


Inhoud

In die dekades sedert die opsporing van kosmiese mikrogolfagtergrond (CMB) in 1965, [10] het die oerknal-model die mees aanvaarde model geword wat die evolusie van ons heelal verklaar. Die Friedmann-vergelyking definieer hoe die energie in die heelal sy uitbreiding dryf.

waar κ die kromming van die heelal voorstel, a(t) is die skaalfaktor, ρ is die totale energiedigtheid van die heelal, en H is die Hubble-parameter. [11]

Ons kan dan die Hubble-parameter herskryf as

waar die vier huidige hipotese bydraers tot die energiedigtheid van die heelal kromming, materie, bestraling en donker energie is. [12] Elk van die komponente neem af met die uitbreiding van die heelal (toenemende skaalfaktor), behalwe miskien die donker-energieterm. Dit is die waardes van hierdie kosmologiese parameters wat fisici gebruik om die versnelling van die heelal te bepaal.

Die versnellingsvergelyking beskryf die evolusie van die skaalfaktor mettertyd

waar die druk P gedefinieer word deur die gekose kosmologiese model. (sien verduidelikende modelle hieronder)

Natuurkundiges was op 'n tyd so verseker van die vertraging van die uitbreiding van die heelal dat hulle 'n sogenaamde vertragingsparameter ingestel het q0 . [13] Huidige waarnemings dui aan dat hierdie vertragingsparameter negatief is.

Verhouding tot inflasie

Volgens die teorie van kosmiese inflasie het die heel vroeë heelal 'n periode van baie vinnige, kwasi-eksponensiële uitbreiding ondergaan. Terwyl die tydskaal vir hierdie periode van uitbreiding baie korter was as dié van die huidige uitbreiding, was dit 'n tydperk van versnelde uitbreiding met 'n paar ooreenkomste met die huidige tydvak.

Tegniese definisie Edit

Om te leer oor die tempo van uitbreiding van die heelal, kyk ons ​​na die grootte-rooi-verskuiwing van astronomiese voorwerpe met behulp van standaard kerse, of hul afstand-rooi-verskuiwing met behulp van standaard liniale. Ons kan ook kyk na die groei van grootskaalse strukture en vind dat die waargenome waardes van die kosmologiese parameters die beste beskryf word deur modelle wat 'n versnelde uitbreiding insluit.

Supernova-waarneming Wysig

In 1998 kom die eerste bewyse vir versnelling van die waarneming van Type Ia-supernovas, wat wit dwerge ontplof wat hul stabiliteitsperk oorskry het. Omdat hulle almal soortgelyke massas het, is hul intrinsieke helderheid standaardiseerbaar. Herhaalde beeldvorming van geselekteerde dele van die lug word gebruik om die supernovas te ontdek. Vervolgwaarnemings gee hul helderheid, wat omgeskakel word in 'n hoeveelheid wat bekend staan ​​as helderheidsafstand (sien afstandsmetings in kosmologie vir besonderhede). [14] Spektrale lyne van hul lig kan gebruik word om hul rooiverskuiwing te bepaal.

Vir supernovas met 'n rooi verskuiwing van minder as ongeveer 0,1, of 'n ligte reistyd van minder as 10 persent van die ouderdom van die heelal, gee dit 'n byna liniêre verhouding – rooiverskuiwing as gevolg van die wet van Hubble. Aangesien die uitbreidingstempo van die heelal oor tyd verander het op groter afstande, wyk die afstand-rooiverskuiwingsverhouding af van lineariteit, en hierdie afwyking hang af van hoe die uitbreidingstempo oor tyd verander het. Die volledige berekening vereis rekenaarintegrasie van die Friedmann-vergelyking, maar 'n eenvoudige afleiding kan soos volg gegee word: die rooi verskuiwing z gee die kosmiese skaalfaktor direk toe die supernova ontplof het.

Baryon akoestiese ossillasies

In die vroeë heelal voordat herkombinasie en ontkoppeling plaasgevind het, bestaan ​​fotone en materie in 'n oerplasma. Punte met 'n hoër digtheid in die foton-barionplasma sou saamtrek en deur swaartekrag saamgepers word totdat die druk te groot geword het en hulle weer uitgebrei het. [13] [ bladsy benodig ] Hierdie inkrimping en uitbreiding het vibrasies in die plasma veroorsaak wat analoog is aan klankgolwe. Aangesien donker materie slegs gravitasie op mekaar inwerk, bly dit in die middel van die klankgolf, die oorsprong van die oorspronklike oordadigheid. Toe ontkoppeling plaasgevind het, ongeveer 380,000 jaar na die oerknal, [17] het fotone van materie geskei en kon hulle vryelik deur die heelal stroom, wat die kosmiese mikrogolfagtergrond skep soos ons dit ken. Dit het skulpe van baryoniese materie in 'n vaste radius gelaat vanaf die oordigtheid van donker materie, 'n afstand wat bekend staan ​​as die klankhorison. Met verloop van tyd en die heelal het uitgebrei, was dit by hierdie anisotropieë van materiaaldigtheid waar sterrestelsels begin vorm het. Deur dus te kyk na die afstande waarop sterrestelsels by verskillende rooi verskuiwings geneig is om te groepeer, is dit moontlik om 'n standaardhoekafstand te bepaal en dit te gebruik om te vergelyk met die afstande wat deur verskillende kosmologiese modelle voorspel word.

Pieke is gevind in die korrelasiefunksie (die waarskynlikheid dat twee sterrestelsels 'n sekere afstand van mekaar sal wees) op 100 h −1 Mpc, [12] (waar h is die dimensielose Hubble-konstante) wat aandui dat dit vandag die grootte van die klankhorison is, en deur dit te vergelyk met die klankhorison tydens ontkoppeling (met behulp van die CMB), kan ons die versnelde uitbreiding van die heelal bevestig. [18]

Clusters of galaxies Edit

Meting van die massafunksies van sterrestelsels, wat die getaldigtheid van die trosse bo 'n drumpelmassa beskryf, lewer ook bewyse vir donker energie [ verdere verduideliking nodig ]. [19] Deur hierdie massafunksies by hoë en lae rooiverskuiwings te vergelyk met die voorspel deur verskillende kosmologiese modelle, waardes vir w en Ωm word verkry wat 'n lae materie-digtheid en 'n nie-donker hoeveelheid donker energie bevestig. [16]

Ouderdom van die heelal

Gegewe 'n kosmologiese model met sekere waardes van die kosmologiese digtheidsparameters, is dit moontlik om die Friedmann-vergelykings te integreer en die ouderdom van die heelal af te lei.

Deur dit te vergelyk met die werklike meetwaardes van die kosmologiese parameters, kan ons die geldigheid van 'n model wat nou versnel, bevestig en in die verlede 'n stadiger uitbreiding gehad het. [16]

Swaartekraggolwe as standaard sirenes

Onlangse ontdekkings van gravitasiegolwe deur LIGO en VIRGO [20] [21] [22] het nie net Einstein se voorspellings bevestig nie, maar ook 'n nuwe venster in die heelal oopgemaak. Hierdie swaartekraggolwe kan werk as 'n soort standaard sirenes om die uitbreidingstempo van die heelal te meet. Abt et al. 2017 het die Hubble-konstante waarde ongeveer 70 kilometer per sekonde per megaparsek gemeet. [20] Die amplitudes van die spanning 'h' hang af van die massas van die voorwerpe wat golwe veroorsaak, afstande vanaf waarnemingspunt en die opsporing van gravitasiegolwe. Die gepaardgaande afstandsmetings is afhanklik van die kosmologiese parameters soos die Hubble Constant vir nabygeleë voorwerpe [20] en sal afhang van ander kosmologiese parameters soos die donker energiedigtheid, materiaaldigtheid, ens. Vir verre bronne. [23] [22]

Donker energie Edit

Die belangrikste eienskap van donker energie is dat dit negatiewe druk (afstootlike werking) het wat relatief homogeen in die ruimte versprei word.

Die eenvoudigste verklaring vir donker energie is dat dit in hierdie geval 'n kosmologiese konstante of vakuumenergie is w = −1. Dit lei tot die Lambda-CDM-model, wat sedert 2003 tot nou toe bekend staan ​​as die Standard Model of Cosmology, aangesien dit die eenvoudigste model is wat goed ooreenstem met 'n verskeidenheid onlangse waarnemings. Riess et al. het bevind dat hul resultate van supernova-waarnemings die uitbreiding van modelle met positiewe kosmologiese konstante bevoordeel ( Ωλ & gt 0) en 'n huidige versnelde uitbreiding ( q0 & lt 0). [15]

Phantom energy Edit

Alternatiewe teorieë Redigeer

Daar is baie alternatiewe verklarings vir die versnelde heelal. Enkele voorbeelde is kwintensie, 'n voorgestelde vorm van donker energie met 'n nie-konstante vergelyking, waarvan die digtheid mettertyd afneem. 'N Negatiewe massakosmologie neem nie aan dat die massadigtheid van die heelal positief is nie (soos in supernova-waarnemings gedoen word), maar vind 'n negatiewe kosmologiese konstante. Occam se skeermes dui ook daarop dat dit die 'meer parsimonieuse hipotese' is. [26] [27] Donker vloeistof is 'n alternatiewe verklaring vir die versnelling van uitbreiding wat poog om donker materie en donker energie in 'n enkele raamwerk te verenig. [28] Alternatiewelik het sommige outeurs aangevoer dat die versnelde uitbreiding van die heelal te wyte kan wees aan 'n afstootlike swaartekraginteraksie van antimaterie [29] [30] [31] of 'n afwyking van die swaartekragwette van algemene relatiwiteit, soos massiewe swaartekrag. , wat beteken dat swaartekragte self massa het. [32] Die meting van die spoed van swaartekrag met die gravitasiegolfgebeurtenis GW170817 het baie gewysigde gravitasieteorieë uitgesluit as alternatiewe verklaring vir donker energie. [33] [34] [35]

'N Ander soort model, die terugreaksie-vermoede, [36] [37] is deur kosmoloog Syksy Räsänen voorgestel: [38] die uitbreidingstempo is nie homogeen nie, maar ons is in 'n gebied waar die uitbreiding vinniger as die agtergrond is. Inhomogeniteite in die vroeë heelal veroorsaak die vorming van mure en borrels, waar die binnekant van 'n borrel minder stof het as gemiddeld. Volgens algemene relatiwiteit is ruimte minder geboë as op die mure, en dit lyk dus asof dit meer volume en 'n hoër uitbreidingstempo het. In die digter streke word die uitbreiding vertraag deur 'n hoër aantrekkingskrag vir swaartekrag. Daarom lyk die innerlike ineenstorting van die digter streke dieselfde as 'n versnelde uitbreiding van die borrels, wat ons lei tot die gevolgtrekking dat die heelal 'n versnelde uitbreiding ondergaan. [39] Die voordeel is dat dit geen nuwe fisika soos donker energie benodig nie. Räsänen beskou die model nie waarskynlik nie, maar sonder enige vervalsing moet dit 'n moontlikheid bly. Dit sal redelik groot digtheidskommelings (20%) vereis om te werk. [38]

'N Laaste moontlikheid is dat donker energie 'n illusie is wat veroorsaak word deur mate van vooroordeel. As ons byvoorbeeld in 'n leër ruimte as die gemiddelde ruimte geleë is, kan die waargenome kosmiese uitbreidingstempo verkeerdelik gesien word as 'n variasie in tyd of versnelling. [40] [41] [42] [43] 'n Ander benadering gebruik 'n kosmologiese uitbreiding van die ekwivalensiebeginsel om aan te toon hoe die ruimte vinniger kan uitbrei in die leemtes rondom ons plaaslike groep. Alhoewel dit swak is, kan sulke effekte wat oor miljarde jare kumulatief beskou word, beduidend word, wat die illusie van kosmiese versnelling skep en dit laat lyk asof ons in 'n Hubble-borrel leef. [44] [45] [46] Nog ander moontlikhede is dat die versnelde uitbreiding van die heelal 'n illusie is wat veroorsaak word deur die relatiewe beweging van ons na die res van die heelal, [47] [48] of dat die grootte van die supernova-monster gebruik word. was nie groot genoeg nie. [49] [50]

Namate die heelal uitbrei, neem die digtheid van straling en gewone donker materie vinniger af as die digtheid van donker energie (sien die vergelyking van die staat) en uiteindelik domineer donker energie. Spesifiek, wanneer die skaal van die heelal verdubbel, word die digtheid van materie met 'n faktor 8 verminder, maar die digtheid van donker energie is byna onveranderd (dit is presies konstant as die donker energie die kosmologiese konstante is). [13] [ bladsy benodig ]

In modelle waar donker energie die kosmologiese konstante is, sal die heelal in die verre toekoms eksponensieel met tyd uitbrei en nader en nader aan 'n de Sitter-heelal kom. Dit sal uiteindelik daartoe lei dat alle bewyse vir die oerknal verdwyn, aangesien die kosmiese mikrogolfagtergrond rooi verskuif word na laer intensiteite en langer golflengtes. Uiteindelik sal die frekwensie daarvan laag genoeg wees dat dit deur die interstellêre medium geabsorbeer sal word, en sodoende gekeur word van enige waarnemer in die sterrestelsel. Dit sal plaasvind wanneer die heelal minder as 50 keer sy huidige ouderdom is, wat lei tot die einde van die kosmologie soos ons dit ken terwyl die verre heelal donker word. [51]

'N Heel konstant uitbreidende heelal met 'n nie-nul-kosmologiese konstante het massadigtheid wat oor tyd afneem. In so 'n scenario is die huidige begrip dat alle materie sal ioniseer en sal disintegreer in geïsoleerde stabiele deeltjies soos elektrone en neutrino's, met alle komplekse strukture wat verdwyn. [52] Hierdie scenario staan ​​bekend as 'hitte-dood van die heelal'.

Alternatiewe vir die uiteindelike lot van die heelal sluit in die Big Rip hierbo genoem, 'n Big Bounce, Big Freeze of Big Crunch.


Antwoorde en antwoorde

Die aantrekkingskrag wissel met die afstand tussen die voorwerpe. As sodanig verander die krag voortdurend en is dit nooit bestendig nie.

As die swaartekrag verband hou met die kromming van die ruimte en as die ruimte uitbrei. Moet daar 'n verdunning plaasvind van alles wat die weefsel van die ruimte uitmaak en dus 'n geleidelike afname in die swaartekrag?

Jy is korrek. Op 'n universele skaal neem die energiedigtheid wat bydra tot swaartekrag af, wat lei tot 'n versnelde uitbreiding namate swaartekrag tussen voorwerpe verswak. AKA beteken dit dat namate die ruimte normale materie uitbrei en donker materie meer en meer geïsoleer word in sterrestelselgroepe met groot ruimtes van intergalaktiese leemtes byna heeltemal leeg. Namate die uitbreiding voortduur, word hierdie leemtes groter en word die erns daarin kleiner. Op 'n universele skaal dra swaartekrag dus al hoe minder by na verloop van tyd.

Sekerlik, daar is variasie van oomblik tot oomblik en die massa van die voorwerpe verander van oomblik tot oomblik. Ek neem aan dat veranderinge in massa die kromming van ruimte beïnvloed en dat swaartekrag nooit bestendig is nie. Op die lange duur kan dit egter as 'n konstante beskou word. Miskien is die verdunning van die ruimte so stadig dat ons nie die vermindering in swaartekrag kan meet nie.

Jy is korrek. Op 'n universele skaal neem die energiedigtheid wat bydra tot swaartekrag af, wat lei tot 'n versnelde uitbreiding namate swaartekrag tussen voorwerpe verswak. AKA beteken dit dat namate die ruimte normale materie uitbrei en donker materie meer en meer geïsoleer word in sterrestelselgroepe met groot ruimtes van intergalaktiese leemtes byna heeltemal leeg. Namate die uitbreiding voortduur, word hierdie leemtes groter en word die erns daarin kleiner. Op 'n universele skaal dra swaartekrag dus al hoe minder by na verloop van tyd.

Hoe sal dit die baan van die son deur die sterrestelsel beïnvloed (ek lees dit neem 225 miljoen jaar om een ​​baan te voltooi)?

Sal die aarde mettertyd groter word en kouer temperature skep? Ek neem aan dat ons sal ontwikkel om so 'n verandering te aanvaar, aangesien ons in die huidige Aardtoestande ontwikkel.


Hoe die Big Crunch Theory werk

Ons is almal bekommerd oor wat aan die einde van ons lewe sal gebeur. Ons sien dat ander lewende dinge sterf, en ons weet dat dit met ons sal gebeur. Omdat dit onvermydelik is, is ons bekommerd oor wanneer, waar en hoe dit gaan gebeur. Baie van ons wonder ook oor die lot van die aarde. Sal dit vir altyd 'n gasvrye blou bal wees, of sal dit uiteindelik deur die son verteer word as dit van 'n mediumgrootte geel ster tot 'n rooi reus opswel? Of miskien vergiftig ons ons planeet, en dit dryf, koud en verlate, deur die ruimte. As so iets sou gebeur, hoe lank sou dit neem? Honderd jaar? N duisend? 'N Miljoen?

Sommige sterrekundiges - diegene wat hulself kosmoloë noem - vra soortgelyke vrae oor die heelal. Die skaal waaraan hierdie wetenskaplikes werk, is natuurlik baie anders. Die heelal is enorm in vergelyking met 'n enkele planeet, selfs 'n enkele sterrestelsel, en sy tydlyn is baie, baie langer. As gevolg hiervan kan kosmoloë nie met sekerheid weet hoe die heelal begin het of hoe dit gaan eindig nie. Hulle kan egter bewyse versamel, opgevoed raai en teorieë opstel.

Een so 'n teorie rakende die toekoms van die heelal staan ​​op speelse wyse bekend as die & quotbig crunch. & Quot Volgens hierdie teorie sal die heelal eendag ophou om uit te brei. Wanneer die swaartekrag die saak tref, sal die heelal dan begin saamtrek en na binne val totdat dit weer in 'n super-warm, super-digte enkelheid teruggeval het. As die teorie geld, is die heelal soos 'n reuse soufflé. Dit begin klein en brei dan uit terwyl dit opwarm. Uiteindelik word die soufflé egter afkoel en begin ineenstort.

Niemand hou van 'n gevalle soufflé nie, en ons moet nie hou van 'n heelal wat soos een optree nie. Dit spel die ondergang van elke sterrestelsel, ster en planeet wat tans bestaan. Gelukkig is die groot verknorsing nie 'n waarborg nie. Kosmoloë is tans besig met 'n warm debat. Die een kamp sê die soufflé sal val, die ander kamp sê die soufflé sal vir ewig uitbrei. Dit sal miljarde jare duur voordat ons met sekerheid weet watter kamp reg is.

Laat ons intussen dieper in die groot nood druk om te verstaan ​​wat dit is en wat dit vir die heelal beteken. Omdat die groot verknorsing eintlik 'n gevolg is van die oerknal, laat ons daar begin.

Alhoewel How the Big Bang Theory Works die oorsprong van die heelal in detail bespreek, sal dit nuttig wees om die basiese beginsels hier te bespreek. Die kort weergawe lui so: Sowat 15 miljard jaar gelede is alle materie en energie in 'n ongelooflike klein streek, bekend as 'n singulariteit. In 'n oomblik het hierdie enkele punt van super-digte materiaal vinnig verbreed. Sterrekundiges verstaan ​​nie heeltemal wat die uitbreiding laat ontstaan ​​het nie, maar hulle gebruik die term & quotbig bang & quot om beide die enkelheid en die eerste paar oomblikke wat daarop volg, te beskryf.

Namate die pasgebore heelal uitgebrei het, het dit begin afkoel en minder dig geword. Dink aan 'n stoomstraal wat uit 'n ketel kom. Naby die tuitdeksel is die stoom redelik warm en die stoommolekules word in 'n beperkte ruimte gekonsentreer. Terwyl die stoom van die ketel af wegbeweeg, word die stoom egter afkoel namate die molekules deur u kombuis versprei. Dieselfde het na die oerknal gebeur. Binne ongeveer 300 000 jaar het alles wat in die enkelheid gehou is, uitgebrei tot 'n siedende, ondeursigtige sfeer van materie en straling. Soos dit wel gebeur het, het die temperatuur gedaal tot 5,432 grade Fahrenheit (3000 grade Celsius), wat meer stabiele deeltjies laat vorm het. Eers het elektrone en protone gekom, wat dan saamgevoeg het om waterstof- en heliumatome te vorm.

Die heelal het voortgegaan om uit te brei en uit te dun. U kan in die versoeking kom om hierdie jong heelal as 'n bredie voor te stel, met klompies materie wat in dik sous dryf. Maar sterrekundiges dink nou dit was meer soos 'n sop, baie glad in digtheid, behalwe 'n paar klein skommelinge. Hierdie versteurings was net betekenisvol genoeg om materie te laat saamvloei. Groot trosse van protogalaxies begin vorm. Die protogalaxies het verouder sterrestelsels, groot eilande van gas en stof wat miljarde sterre gebaar het. Rondom sommige van die sterre het swaartekrag rotse, ys en ander materiale saamgetrek om planete te vorm. Op ten minste een van daardie planete het die lewe ontwikkel, ongeveer 11 miljard jaar nadat die oerknal alles begin het.

Vandag brei die heelal voort, en sterrekundiges het bewyse om dit te bewys. Vervolgens gaan ons sommige van die bewyse ondersoek.

Bewyse vir die oerknal

As die oerknalteorie korrek is, moet sterrekundiges die uitbreiding van die heelal kan opspoor. Edwin Hubble, die naamgenoot van die Hubble-ruimteteleskoop, was een van die eerste wetenskaplikes wat hierdie uitbreiding waargeneem en gemeet het. In 1929 studeer hy die spektra, of reënboë, van verre sterrestelsels deur die lig van hierdie voorwerpe deur 'n prisma op sy teleskoop te laat gaan. Hy het opgemerk dat die lig van byna elke sterrestelsel na die rooi punt van die spektrum verskuif. Om die waarneming te verduidelik, wend hy hom na die Doppler-effek, 'n verskynsel wat die meeste mense met klank assosieer. As 'n ambulans ons byvoorbeeld op straat nader, lyk dit asof die toonhoogte van die sirene toeneem namate dit verbygaan, die toonhoogte afneem. Dit gebeur omdat die ambulans die klankgolwe wat dit skep, inhaal (verhoogde toonhoogte) of wegbeweeg van hulle af (verminderde toonhoogte).

Hubble het geredeneer dat liggolwe wat deur sterrestelsels geskep word, dieselfde optree. As 'n verre sterrestelsel na ons sterrestelsel toe jaag, beweer hy, sal dit nader aan die liggolwe beweeg wat dit produseer, wat die afstand tussen golwings verminder en die kleur na die blou punt van die spektrum sal verskuif. As 'n verre sterrestelsel van ons sterrestelsel af wegjaag, sou dit wegbeweeg van die liggolwe wat dit geskep het, wat die afstand tussen golwings verhoog en die kleur na die rooi punt van die spektrum sou skuif. Nadat hy deurgaans rooiverskuiwings waargeneem het, het Hubble ontwikkel wat ons noem Hubble se wet: Sterrestelsels beweeg van ons af teen 'n snelheid wat eweredig is aan hul afstand van die aarde af.

Vandag is die rooi verskuiwings van hemelse voorwerpe 'n sterk bewys dat die heelal uitbrei. Maar alles wat uitbrei, moet uiteindelik stop, of hoe? Sal die heelal nie, net soos 'n bal wat in die lug gegooi word, die een of ander maksimum punt van uitbreiding bereik, stop en dan begin terugval na waar dit begin het nie? Soos ons verder gaan sien, is dit een van drie moontlike scenario's.

Sterk bewyse vir die oerknal kom ook van die kosmiese mikrogolf-agtergrond (CMB) -straling. Hierdie mikrogolwe is dieselfde as wat u in u kombuis kos gebruik, behalwe dat dit deur die hele wêreld versprei is. In werklikheid is hulle so eweredig deur die ruimte versprei dat sterrekundiges nou glo dat die CMB-straling die eggo van die oerknal is, die sterwende snak van die ontploffing wat die kosmos wat ons vandag ken, gebaar het.

Byna alle sterrekundiges aanvaar dat die heelal besig is om uit te brei. Wat volgende gebeur, is die ware raaisel. Gelukkig is daar net drie werklike moontlikhede: die heelal kan oop, plat of toe wees.

Oop Heelal. In hierdie scenario sal die heelal vir ewig uitbrei, en soos dit ook gebeur, sal die materiaal wat dit bevat, dunner en dunner versprei. Uiteindelik sal sterrestelsels opraak met die grondstowwe wat hulle nodig het om nuwe sterre te maak. Sterre wat reeds bestaan, sal stadigaan uitdoof, soos sterwende gloed. In plaas van vurige wiegies, word sterrestelsels kiste gevul met stof en dooie sterre. Op daardie stadium sal die heelal donker, koud en helaas leweloos word.

Plat Heelal. Stel jou voor dat 'n albaster op 'n oneindig lang houtoppervlak rol. Daar is net genoeg wrywing om die albaster te vertraag, maar nie genoeg om dit vinnig te doen nie. Die marmer sal lank rol en uiteindelik stadig en sag stop. Dit is wat met 'n plat heelal sal gebeur. Dit sal al die energie van die oerknal verbruik en, tot in ewewig, tot stilstand kom tot ver in die toekoms. Op baie maniere is dit net 'n variasie van die oop heelal, want dit sal letterlik vir ewig neem voordat die heelal die ewewigspunt bereik.

Geslote heelal. Bind die een punt van 'n riem aan jou been, die ander punt aan die spoor van 'n brug en spring dan weg. U sal vinnig afwaarts versnel totdat u die koord begin rek. Namate spanning toeneem, vertraag die koord geleidelik u afkoms. Uiteindelik sal u heeltemal tot stilstand kom, maar net vir 'n oomblik terwyl die koord, tot sy uiterste gerek, u terugskuif na die brug. Sterrekundiges dink dat 'n geslote heelal op dieselfde manier sal optree. Die uitbreiding daarvan sal vertraag totdat dit 'n maksimum grootte bereik. Dan sal dit terugsak en op homself ineenstort. Soos dit wel gebeur, sal die heelal digter en warmer word totdat dit eindig in 'n oneindig warm, oneindig digte singulariteit.

'N Geslote heelal sal lei tot 'n groot verknorsing - die teenoorgestelde van die oerknal. Maar wat is die kans dat 'n geslote heelal waarskynliker is as 'n oop of plat heelal? Sterrekundiges begin met 'n paar opgeleide raaiskote.

Om te bepaal of die heelal vir ewig sal uitbrei, tot stilstand kom of op homself ineenstort, moet sterrekundiges besluit watter een van die twee opponerende magte 'n kosmiese toutrekkery sal wen. Een van hierdie kragte is die knaldeel van die oerknal - die ontploffing wat die heelal in alle rigtings na buite gekatapuleer het. Die ander krag is swaartekrag, die trek wat een voorwerp op 'n ander uitoefen. As die swaartekrag binne die heelal sterk genoeg is, kan dit in die uitbreiding regeer en die heelal laat saamtrek. Indien nie, sal die heelal vir altyd aanhou uitbrei.

Alhoewel sterrekundiges weet dat die heelal uitbrei, kan hulle nie die krag wat verantwoordelik is vir die uitbreiding presies meet nie. In plaas daarvan probeer hulle om die digtheid van die heelal te meet. The higher the density, the greater the gravitational force. Applying this logic, there must be a density threshold -- a critical limit -- that will determine if the gravity within the universe is strong enough to halt the expansion and reel everything back in. If the density is greater than the critical limit, then the universe will stop expanding and start contracting. If it's less than the critical limit, then the universe will expand forever. Astronomers represent this mathematically with the following equation:

Ω = actual average density/critical density

If omega (Ω) is greater than 1, then the universe will be closed. If it's less than 1, the universe will be open. And if it's equal to 1, the universe will be flat. Based on the matter we can see, such as galaxies, ­stars and planets, the density of the universe seems to be below the critical value. This would suggest an open universe that will expand forever. But cosmologists think there is another type of matter that can't be seen. Dit dark matter may account for much more of the universe than ordinary, visible matter and may have enough gravity to stop, and then reverse, the expansion.

Recently, astronomers have made some observations that indicate there's another invisible material in the cosmos: dark energy. Could dark energy profoundly affect the universe's fate?

The term "big bang" started as a joke -- a derogatory remark made by astronomer Fred Hoyle. But the name stuck and spawned a series of nomenclature knockoffs. A universe that expands forever will yield a "big chill" or a "big freeze." A universe that collapses into a singularity and explodes outward again will experience a "big crunch" followed by a "big bounce." And a universe that reaches equilibrium and does nothing will become a "big bore."

­Just as astronomers were grappling with the impact of dark matter, they made a discovery that caused them to go back to the chalkboard once again. The discovery came in 1998, when the world's best telescopes revealed that type Iasupernovae -- dying stars that all have the same intrinsic brightness -- were farther away from our galaxy than they should have been. To explain this observation, astronomers suggested that the expansion of the universe is actually accelerating or speeding up. But what would cause the expansion to go faster? Isn't the gravity inherent in dark matter strong enough to prevent such an expansion?

As it turns out, there's more to the cosmic story than previously thought. Some cosmologists now think that something else -- something just as inexplicable and unobservable as dark matter -- is lurking in the universe. They sometimes refer to this invisible stuff as dark energy. Unlike gravity, which pulls on the universe and slows its expansion, dark energy pushes on the universe and works to speed up the expansion. And there's a lot of it. Astronomers estimate that the universe might be 73 percent dark energy. Dark matter, they think, makes up another 23 percent, and ordinary matter -- the stuff we can see -- makes up a paltry 4 percent [source: Brecher]. With numbers like that and given that dark energy is an inflationary force, it's easy to see how the big crunch might never happen at all.

Interestingly, Albert Einstein predicted the existence of dark energy back in 1917 as he tried to balance the equations of his general theory of relativity. He didn't call it dark energy at the time. He referred to it as the cosmological constant and labeled it lambda in his calculations. Although he couldn't prove it, Einstein thought there must be a repulsive force in the universe to spread everything around so evenly. Eventually, he recanted, calling lambda his greatest blunder.

­Now, scientists are wondering if Einstein may have been right once again -- unless, of course, he's wrong. Up next, we'll explore why some still hold the big crunch in high regard and why it might not be the end of the universe, but a second beginning.

Clearly, there's no easy answer when it comes to predicting the fate of the universe. But let's imagine for a ­moment th­at the density of the universe is above the critical value required to stop expansion. This would lead to the big crunch, which in many ways would be like hitting the rewind button on a VCR. As gravity within the universe pulled everything back, galaxy clusters would draw closer together. Then individual galaxies would begin to merge until, after billions of years, one mega-galaxy would form.

Inside this gigantic cauldron, stars would meld together, causing all of space to become hotter than the sun. Eventually, stars would explode and black holes would emerge, slowly at first and then more rapidly. As the end drew near, the black holes would suck up everything around them. Even they would coalesce at some point to form a monstrous black hole that would pull the universe closed like a drawstring bag. At the end, nothing would remain but a super-hot, super-dense singularity -- the seed of another universe. Many astronomers think the seed would germinate in a "big bounce," starting the whole process over again.

That's not the only theory. A few cosmologists, led by Paul J. Steinhardt of Princeton University and Neil Turok of Cambridge University, have recently argued that the big chill and the big crunch are not mutually exclusive. Their model works like this: The universe began with the big bang, which was followed by a period of slow expansion and gradual accumulation of dark energy. This is where we are today. What happens next is highly speculative, but Steinhardt and Turok believe that the dark energy will continue to accumulate and, as it does, will stimulate cosmic acceleration. The universe won't ever stop expanding, but will spread out over trillions of years, stretching all matter and energy to such an extreme that our one universe will be separated into multiple universes. Inside these universes, the mysterious dark energy will materialize into normal matter and radiation. This will trigger another big bang -- perhaps several of them -- and another cycle of expansion.

­If you're disconcerted by all this talk of crunching and expanding, you can take comfort in knowing that the fate of the universe won't be determined for billions, maybe even trillions, of years. That gives you plenty of time to focus on things that are a bit more certain, such as your own life cycle of birth, growth and death.


Explaining the accelerating expansion of the universe without dark energy

A still from an animation that shows the expansion of the universe in the standard 'Lambda Cold Dark Matter' cosmology, which includes dark energy (top left panel, red), the new Avera model, that considers the structure of the universe and eliminates the need for dark energy (top middle panel, blue), and the Einstein-de Sitter cosmology, the original model without dark energy (top right panel, green). The panel at the bottom shows the increase of the 'scale factor' (an indication of the size) as a function of time, where 1Gya is 1 billion years. The growth of structure can also be seen in the top panels. One dot roughly represents an entire galaxy cluster. Units of scale are in Megaparsecs (Mpc), where 1 Mpc is around 3 million million million km. Credit: István Csabai et al

Enigmatic 'dark energy', thought to make up 68% of the universe, may not exist at all, according to a Hungarian-American team. The researchers believe that standard models of the universe fail to take account of its changing structure, but that once this is done the need for dark energy disappears. The team publish their results in a paper in Monthly Notices of the Royal Astronomical Society.

Our universe was formed in the Big Bang, 13.8 billion years ago, and has been expanding ever since. The key piece of evidence for this expansion is Hubble's law, based on observations of galaxies, which states that on average, the speed with which a galaxy moves away from us is proportional to its distance.

Astronomers measure this velocity of recession by looking at lines in the spectrum of a galaxy, which shift more towards red the faster the galaxy is moving away. From the 1920s, mapping the velocities of galaxies led scientists to conclude that the whole universe is expanding, and that it began life as a vanishingly small point.

In the second half of the twentieth century, astronomers found evidence for unseen 'dark matter' by observing that something extra was needed to explain the motion of stars within galaxies. Dark matter is now thought to make up 27% of the content of universe (in contrast 'ordinary' matter amounts to only 5%).

Observations of the explosions of white dwarf stars in binary systems, so-called Type Ia supernovae, in the 1990s then led scientists to the conclusion that a third component, dark energy, made up 68% of the cosmos, and is responsible for driving an acceleration in the expansion of the universe.

In the new work, the researchers, led by Phd student Gábor Rácz of Eötvös Loránd University in Hungary, question the existence of dark energy and suggest an alternative explanation. They argue that conventional models of cosmology (the study of the origin and evolution of the universe), rely on approximations that ignore its structure, and where matter is assumed to have a uniform density.

"Einstein's equations of general relativity that describe the expansion of the universe are so complex mathematically, that for a hundred years no solutions accounting for the effect of cosmic structures have been found. We know from very precise supernova observations that the universe is accelerating, but at the same time we rely on coarse approximations to Einstein's equations which may introduce serious side-effects, such as the need for dark energy, in the models designed to fit the observational data." explains Dr László Dobos, co-author of the paper, also at Eötvös Loránd University.

In practice, normal and dark matter appear to fill the universe with a foam-like structure, where galaxies are located on the thin walls between bubbles, and are grouped into superclusters. The insides of the bubbles are in contrast almost empty of both kinds of matter.

Using a computer simulation to model the effect of gravity on the distribution of millions of particles of dark matter, the scientists reconstructed the evolution of the universe, including the early clumping of matter, and the formation of large scale structure.

Unlike conventional simulations with a smoothly expanding universe, taking the structure into account led to a model where different regions of the cosmos expand at different rate. The average expansion rate though is consistent with present observations, which suggest an overall acceleration.

Dr Dobos adds: "The theory of general relativity is fundamental in understanding the way the universe evolves. We do not question its validity we question the validity of the approximate solutions. Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion. These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy."

If this finding is upheld, it could have a significant impact on models of the universe and the direction of research in physics. For the past 20 years, astronomers and theoretical physicists have speculated on the nature of dark energy, but it remains an unsolved mystery. With the new model, Csabai and his collaborators expect at the very least to start a lively debate.


Dark future

Waves from photons, electrons, positrons and neutrinos would be the last things in the universe, under the Big Freeze scenario

Projecting the various dark energy theories into the future, the universe could be in for a suitably dark end.

If the expansion keeps accelerating, eventually all the galaxies in the sky would vanish from view. That’s because at a certain point, they’d be moving away from us faster than the light they emit would travel towards us, ensuring that this light could never, ever reach us.

Dark energy would continue pushing the universe apart long after every last star has collapsed into a black hole, and every last black hole has evaporated into nothing. Eventually all particles would be spread so far apart that they would rarely meet. The universe would cool down until it has absolutely no thermodynamic free energy. This is known as the Big Freeze.

Different models have different endings. “Phantom energy” is a variation on quintessence, where its density actually increases as the universe expands (as opposed to most models of dark energy where the density stays constant). That means that eventually, this phantom energy will end up overpowering all other fundamental forces in physics.

In this scenario, at some point gravity would become too weak to hold galaxies together. A few dozen million years later and the strong and weak nuclear forces begin to fail, tearing apart stars and planets, and then atoms themselves. Finally, even the fabric of spacetime will rip apart, in an end fittingly called the Big Rip.

Or, if dark energy isn’t constant over time, there’s a chance that in the distant future gravity could win the tug-o’-war and begin pulling everything back together. Over billions of years, the universe would contract until it reaches a point of infinite density, like a reverse Big Bang. This scenario is called the Big Crunch.

Or perhaps, that isn’t the end but a new beginning, as another universe explodes out of the ashes in a so-called Big Bounce.


Data discrepancies may affect understanding of the universe

Why the expansion of the universe appears to be accelerating remains a mystery, but new research from UT Dallas may help shed light on it. Credit: NASA, ESA and the LEGUS team

One of the unsolved mysteries in modern science is why the expansion of the universe appears to be accelerating. Some scientists argue it is due to a theoretical dark energy that counteracts the pull of gravity, while others think Albert Einstein's long-accepted theory of gravity itself may need to be modified.

As astrophysicists look for answers in the mountains of data gathered from astronomical observations, they are finding that inconsistencies in that data might ultimately lead to the truth.

"This is like a detective story, where inconsistent evidence or testimony could lead to solving the puzzle," said Dr. Mustapha Ishak-Boushaki, professor of astrophysics in the School of Natural Sciences and Mathematics at The University of Texas at Dallas.

Ishak-Boushaki and his doctoral student Weikang Lin have developed a new mathematical tool that identifies and quantifies inconsistencies in cosmological data gathered by various scientific missions and experiments. Their findings could shed light on the cosmic acceleration conundrum and have a significant impact on our understanding of the universe.

Their most recent research, published last October in the journal Physical Review D, was presented June 4 at a meeting of the American Astronomical Society in Denver.

"The inconsistencies we have found need to be resolved as we move toward more precise and accurate cosmology," Ishak-Boushaki said. "The implications of these discrepancies are that either some of our current data sets have systematic errors that need to be identified and removed, or that the underlying cosmological model we are using is incomplete or has problems."

Astrophysicists use a standard model of cosmology to describe the history, evolution and structure of the universe. From this model, they can calculate the age of the universe or how fast it is expanding. The model includes equations that describe the ultimate fate of the universe—whether it will continue expanding, or eventually slow down its expansion due to gravity and collapse on itself in a big crunch.

There are several variables—called cosmological parameters—embedded in the model's equations. Numerical values for the parameters are determined from observations and include factors such as how fast galaxies move away from each other and the densities of matter, energy and radiation in the universe.

But there is a problem with those parameters. Their values are calculated using data sets from many different experiments, and sometimes the values do not agree. The result: systematic errors in data sets or uncertainty in the standard model.

"Our research is looking at the value of these parameters, how they are determined from various experiments, and whether there is agreement on the values," Ishak-Boushaki said.

New Tool Finds Inconsistencies

The UT Dallas team developed a new measure, called the index of inconsistency, or IOI, that gives a numerical value to the degree of discordance between two or more data sets. Comparisons with an IOI greater than 1 are considered inconsistent. Those with an IOI over 5 are ranked as strongly inconsistent.

For example, the researchers used their IOI to compare five different techniques for determining the Hubble parameter, which is related to the rate at which the universe is expanding. One of those techniques—referred to as the local measurement—relies on measuring the distances to relatively nearby exploding stars called supernovae. The other techniques rely on observations of different phenomena at much greater distances.

"We found that there is an agreement between four out of five of these methods, but the Hubble parameter from local measurement of supernovae is not in agreement. It's like an outlier," Ishak-Boushaki said. "In particular, there is a clear tension between the local measurement and that from the Planck science mission, which characterized the cosmic microwave background radiation."

To complicate matters, multiple methods have been used to determine that local measurement, and they all produced a similar Hubble value, still in disagreement with Planck and other results.

"Why does this local measurement of the Hubble parameter stand out in significant disagreement with Planck?" Ishak-Boushaki asked.

He and Lin also applied their IOI tool to five sets of observational data related to the large-scale structure of the universe. The cosmological parameters calculated using those five data sets were in strong disagreement, both individually and collectively, with parameters determined by observations from Planck.

"This is very intriguing. This is telling us that the universe at the largest observable scales may behave differently from the universe at intermediate or local scales," Ishak-Boushaki said. "This leads us to question whether Albert Einstein's theory of gravity is valid all the way from small scales to very large scales in the universe."

The UT Dallas researchers have made their IOI tool available for other scientists to use. Ishak-Boushaki said the Dark Energy Science Collaboration, part of the Large Synoptic Survey Telescope project, will use the tool to look for inconsistencies among data sets.

"These inconsistencies are starting to show up more now because our observations have progressed to a level of precision where we can see them," said Ishak-Boushaki, who published his first paper about the inconsistencies in 2005. "We need the right values for these cosmological parameters because it has important implications for our understanding of the universe."


Gravity causes homogeneity of the universe

During its expansion, the universe evolved towards its present state, which is homogeneous and isotropic on large scales. This is inferred, among other things, from the measurement of the so-called background radiation as nicely seen in the full sky image of the WMAP data. New results published in the renowned journal PRL show that homogenization in the investigated class of cosmological models is already completely explained by Einstein's theory of gravity and does not require any additional modifications. Credit: NASA / WMAP Science Team

Gravity can accelerate the homogenization of space-time as the universe evolves. This insight is based on theoretical studies of the physicist David Fajman of the University of Vienna. The mathematical methods developed within the research project allow to investigate fundamental open questions of cosmology such as why the universe today appears so homogeneous. The results have been published in the journal Physical Review Letters.

The temporal evolution of the universe, from the Big Bang to the present, is described by Einstein's field equations of general relativity. However, there are still a number of open questions about cosmological dynamics, whose origins lie in supposed discrepancies between theory and observation. One of these open questions is: Why is the universe in its present state so homogeneous on large scales?

From the Big Bang to the present

It is assumed that the universe was in an extreme state shortly after the Big Bang, characterized in particular by strong fluctuations in the curvature of spacetime. During the long process of expansion, the universe then evolved towards its present state, which is homogeneous and isotropic on large scales—in simple terms: the cosmos looks the same everywhere.

This is inferred, among other things, from the measurement of the so-called background radiation, which appears highly uniform in every direction of observation. This homogeneity is surprising in that even two regions of the universe that were causally decoupled from each other—i.e., they could not exchange information—still exhibit identical values of background radiation.

To resolve this supposed contradiction, the so-called inflation theory was developed, which postulates a phase of extremely rapid expansion immediately after the Big Bang, which in turn can explain the homogeneity in the background radiation.

However, how this phase can be explained in the context of Einstein's theory requires a number of modifications of the theory, which seem artificial and cannot be verified directly.

New findings: Homogenization by gravitation

Up to now, it was not clear whether the homogenization of the universe can be explained completely by Einstein's equations. The reason for this is the complexity of the equations and the associated difficulty to analyze their solutions—models for the universe—and to predict their behavior.

In the concrete problem, the time evolution of the originally strong deviations from the homogeneous state as cosmological gravitational waves has to be analyzed mathematically. It has to be shown that they decay in the course of the expansion thus allowing the universe to get its homogeneous structure.

Such analyses are based on modern mathematical methods in the field of geometric analysis. Until now, these methods could only achieve such results for small deviations from the homogeneous space-time geometry. David Fajman from the University of Vienna has now succeeded for the first time to transfer these methods to the case of arbitrarily large deviations.

The results published in the renowned journal PRL show that homogenization in the investigated class of models is already completely explained by Einstein's theory and does not require any additional modifications. If this finding can be transferred to more general models, it means that it does not necessarily need a mechanism like inflation to explain the state of our present universe, but that Einstein's theory could finally triumph once again.


Gravity and Acceleration

The drawback to Einstein’s Special Theory of Relativity, however, is that it is “special” in the respect that it only considers the effects of relativity to an observer moving at constant speed. Motion at constant speed is clearly a very special case, and in practice, bodies change their speed with time. Einstein wanted to generalize his theory to consider how a person sees another person who is accelerating relative to them.

At around this time (1907), he also started to wonder how Newtonian gravitation would have to be modified to fit in with special relativity, and how the effects of gravity could be incorporated into the formulation. His resulting General Theory of Relativity, over ten years in the making (it was published in 1916), has been called the greatest contribution to science by a single human mind.

Initially, Einstein had been puzzled by the fact that Sir Isaac Newton’s Law of Universal Gravitation, which had stood undisputed since 1687, appeared to be fundamentally incompatible with his own Special Theory. Newton’s theory (permanently linked, at least in the popular mind, with his observation of an apple falling from a tree) stated that every massive body exerts an attractive force on every other massive body, a force which is proportional to the product of the two masses and inversely proportional to the square of the distance between the bodies.


(Click for a larger version)
Newton's Law of Universal Gravitation
(Source: Astronomy Notes: http://www.astronomynotes.com/
gravappl/s3.htm)

Thus, according to this theory, gravity is relatively strong when objects are near each other, but weakens with distance, and the bigger the bodies, the more their force of mutual attraction. This “inverse-square law” is quite sophisticated enough to explain why a cannonball fired horizontally travels further before hitting the ground the faster it is launched, why a certain minimum speed (about 11.2 kilometers per second) would be required to allow objects to break out of Earth’s gravity and into orbit and why the planets travel in an elliptical orbit around the Sun (although not quite sophisticated enough to predict the slight anomaly in Mercury’s orbit).

Gravity is the organizing force for the cosmos, crucial in allowing structure to unfold from an almost featureless Big Bang origin. Although it is a very weak force (feebler than the other fundamental forces which govern the sub-atomic world by a factor of 10 36 or 1,000,000,000,000,000,000,000,000,000,000,000,000), it is a cumulative and consistent force which acts on everything and can act over large distances. So, even though gravity can be effectively ignored by chemists studying how groups of atoms bond together, for bodies more massive than the planet Jupiter the effects of gravity overwhelm the other forces, and it is largely responsible for building the large-scale structures in the universe. Thus, gravity squeezes together massive bodies like our own Sun, and it is only the explosive outward energy in the Sun’s ultra-hot core that holds it in hydrostatic equilibrium and stops it from collapsing into a super-dense white dwarf star.

Even before Newton, the great 17th Century Italian physicist Galileo Galilei had shown that all bodies fall at the same rate, any perceived differences in practice being caused by differences in air resistance and drag. Galileo’s famous (and probably apocryphal) experiment involving the dropping of two balls of different masses from the Leaning Tower of Pisa was repeated with even more dramatic results in 1972 when a hammer and a feather were dropped together on the airless Moon and, just as Galileo had predicted, both hit the ground together.

Newton, however, had assumed that the force of gravity acts instantaneously, and Einstein had already shown that nothing can travel at infinite speed, not even gravity, being limited by the de facto universal speed limit of the speed of light. Furthermore, Newton had assumed that the force of gravity was purely generated by mass, whereas Einstein had shown that all forms of energy had effective mass and must therefore also be sources of gravity.


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The principle of equivalence says that gravity is not a force at all, but is in fact the same thing as acceleration
(Source: Time Travel Research Center: http://www.zamandayolculuk.com/cetinbal/
HTMLdosya1/RelativityFile.htm)

Einstein’s ground-breaking realization (which he called “the happiest thought of my life”) was that gravity is in reality not a force at all, but is indistinguishable from, and in fact the same thing as, acceleration, an idea he called the “principle of equivalence”. He realized that if he were to fall freely in a gravitational field (such as a skydiver before opening his parachute, or a person in an elevator when its cable breaks), he would be unable to feel his own weight, a rather remarkable insight in 1907, many years before the idea of freefall of astronauts in space became commonplace.

A simple thought experiment serves to clarify this: if an astronaut in the cabin of a spacecraft accelerating upwards at 9.8 meters per second per second (the same acceleration as gravity imparts to falling bodies near the Earth’s surface) were to drop a feather and hammer they too would hit the floor of the cabin simultaneously (in the absence of air resistance), exactly as would have happened if they had fallen on Earth under gravity. That, and the feeling of his feet being glued to the ground just as they would be in Earth’s gravity, would be enough to convince the astronaut that the acceleration of the spaceship was indistinguishable from the pull of gravity on the Earth.

The influence of gravity also creates effects of time dilation (see the section on the Special Theory of Relativity for a more detailed discussion of time dilation), sometimes referred to as "gravitational time dilation". As Einstein predicted, the closer a body is to a large mass, with a commensurately large gravitational pull, the slower time runs for it. It is almost as though gravity is pulling on time itself, slowing its progress. Gravitational time dilation also raises the theoretical possibility of time travel. For example, if a spaceship were to orbit close enough (but not too close!) to a hugely massive object such as a supermassive black hole, the gravitational effects may be significant enough to slow down time for the occupants compared to elsewhere, effectively allowing them to travel into the future.

On a much smaller scale, because gravity is slightly stronger closer to the center of the Earth, then theoretically time passes more slowly for someone living on the first floor of an apartment block than for someone living at the top. With modern atomic clocks of sufficient accuracy, differences in the passage of time at different altitudes above sea level (and therefore different distances from the Earth's center of gravity) can be measured, and even the tiny differences due to the changing shape of the Earth as the tidal force of the Moon pulls and stretches it. A real-life example of gravitational time dilation can be seen in GPS systems in geosynchronous orbits above the Earth, which need to constantly adjust their clocks to account for time differences due to the weaker gravity they experience compared to that on the Earth's surface (it is estimated that their accuracy would be out by as much as 10 kilometers a day without this adjustment).

So, gravity, Einstein realized, is not really a force at all, but just the result of our surroundings accelerating relative to us. Or, perhaps a better way of looking at it, gravity is a kind of inertial force, in the same way as the so-called centrifugal force is not a force in itself, merely the effect of a body’s inertia when forced into a circular path. In order to rationalize this situation, though, Einstein was to turn our whole conception of space on its head, as we will see in the next section.