From the Big Bang to the Present: Gravity Is Why the Universe Is So Uniform

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.

Alternative theories

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.

Reference: “Future Attractors in 2+1 Dimensional Gravity” by David Fajman, 16 September 2020, Physical Review Letters.
DOI: 10.1103/PhysRevLett.125.121102

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.


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  • I know this may seem a stupid question - but has hanyone considered what effect enatnglement would have upon the entropy of the early universe?

    • It is not a stupid question!

      Since we are talking about the era before the hot big bang that today causes the universe to be homogeneous and isotropic (as well as flat) - which an expansion cannot do [c.f. ; script from an astrophysicist ] - I am going to assume we are discussing how that happened and how that affected entropy.

      If we take the current inflationary hot big bang cosmology [see the video link] first, the rapid expansion put the now causally disconnected universe into causal contact [see the link in my onw response to the article]. The figure a bit down tries to illustrate the process (though it is hard to grok form it so you may want to read some of the listed references): "The plot illustrates how the perturbation mode grows larger than the horizon during cosmological inflation before coming back inside the horizon, which grows rapidly during radiation domination. If cosmological inflation had never happened, and radiation domination continued back until a gravitational singularity, then the mode would never have been inside the horizon in the very early universe, and no causal mechanism could have ensured that the universe was homogeneous on the scale of the perturbation mode."

      And since inflation had emptied the now observable universe it happened in a 0 K temperature and 0 J/K entropy universe (consistent with the 0 J*m^-3 energy density space of a flat universe - the gravitational and inflationary potential energies must have cancelled).

      Nopw the question becomes if entanglement could be part of, or replace, any of that?

      I don't know how to approach the application of entanglement on homogeneity and isotropy (or flatness!) formally. But your question seems to me to give the method to give a qualitative answer. I am fairly certain that entanglement, which causes entanglement entropy ["Entropy of entanglement", Wikipedia] do not force the universe to have low entropy - it would add entropy. So that doesn't seem to work.

      And since entanglement results in correlations and do not carry light cone relativistic signals, I don't think it can replace the "causal mechanism" that "could have ensured that the universe was homogeneous on the scale of [a] perturbation mode".

  • Hello Bob, I personaly also think that entanglement has played o role, probably a major one, a the beginning of the universe. The Fact that the universe is governed, everywehre , wether causaly connected today or not, by the same laws, can only and must be explained by some form of entanglement at the very beginning, when things still where causaly connected.
    Besides, today things are causaly connected in the observable universe. But what does that mean ? For us, here on earth, the observable universe extends in all direction, like a sphere, and we are at the center. Now let's go, in our minds, at a point close to " frontier " of this sphere. Well, that point will be the center of its own observable universe, from which we are part. When seeing things in that way, one realyses that that the universe is in its totality, yes totality, directly or indirectly, causaly connected.Same laws, spatial homogeniity !

    • The reason why we may have the same laws throughout the observable universe can be expanded to ask how large volumes that applies to, a question that can be given a tentative answer from our observations of inflation. But that is irrelevant to the question of the observable universe, which is given an answer from the current cosmology and likely not from entanglement, see details in my own response to Bob.

      The same reference links I discuss in that comment also explains that we don't see a causally connected universe today. With radiation reaching us from both sides of the universe in respect to us, radiation emitted close to the hot big bang era at the start of the observable universe in its current form, how could it be? The radiation has only had time to reach us across the radius of the universe - it is how the radiation sphere of the cosmic background is defined, by the photons reaching us now - and not to reach across the twice as large diameter. In respect to us we hence know that diametrically opposed volumes when the cosmic background radiation were causally disconnected.

      We have the same laws, but that isn't what caused the homogeneity and isotropy (and flatmess on average everywhere) of the result, c.f. how just an expansion would have given the same laws but not the latter result.

      • "In respect to us we hence know that diametrically opposed volumes when the cosmic background radiation were causally disconnected" - In respect to us, we hence know that diametrically opposed volumes of when the cosmic background radiation were emitted, were causally disconnected.

  • Ironically the simplified toy model is artificial, do not apply to our universe and cannot be verified directly.

    The irony comes from this: "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."

    Inflation is generally accepted today, thanks to that it looks natural (observations describe a quantum field theory) and has been verified on many unique characteristics - such as flatness of space, forgotten above. "The basic inflationary paradigm is accepted by most physicists, as a number of inflation model predictions have been confirmed by observation; ..." [ ].

  • It would seems that if everything started out from a singularity then everything will behave the same unless it is influenced by something else. Entanglement is not different. Simply deterministic

    • The idea that the universe started from a singularity may be fashionable, but it has been rejected for decades now - see the video I linked to in response to Bob for a current description.

      State changes are deterministic in quantum physics, while wave collapse isn't, so you can take your pick. I.e. the entanglement state is deterministic, but it won't help you send a signal due to the random outcome of you observing it. Since inflation has no entanglement (entropy is zero) I don't think it applies, but in any case any entanglement can't feasibly survive the hot big bang era when particles started to interact. The question then becomes, why is the universe after the hot big bang homogeneous and isotropic to 1 part of 100,000 in energy density? The observed inflation has that as a natural consequence (as the nonhomogenities derive from the quantum fluctuations in the inflation field).

    • Erroneous superstition. But the reason we know no 'gods' exist is interesting precisely in this context, since the average flatness of the universe over sufficiently large volumes - which goes with the homogeneity and isotropy due to inflation - means the universe is zero energy.

      Hence we can observe any deviations caused by non-natural means - and there is none. And it is also then easy to approximate the relativistic process with a classical thermodynamic adiabatic free expansion - meaning there was, is and will be no magic actions (such as from superstition 'gods').

      And, to add the obvious: the model here, mistaken as it likely is, is based on science, the only known method to derive knowledge and objective answers. It is by science we now know the natural process and, on average, the objects of the process that the universe undergoes, to 1 % uncertainty at the moment but with ever increasing precision. Even if we did not sufficiently knew that magic do not operate in the universe - which is all there is according to the self contained relativistic model of it - the small remaining slot that existed before the 2018 discovery of average space flatness would have tempt me to paraphrase Hulk from the movie "Avengers": "Puny gods."

  • At the risk of being crude and superficial my admittedly naive reasing is that the disorder in a system composed of entangled particles must be less than an otherwise identical system of non-entangled ones - more so if entanglement applies to more than one feature of each pair of particles. Since high temperatures can promote the creation of entangled pairs, ought we expect a hot-big bang to produce a rather less variable universe than one where the state of every partice therein was independent? Or am I missing something very obvious...

  • ... So!
    How many times modern physics cries out for the God, this days?
    Okay, don't mention entanglement, that makes Albert look funny...

  • ... and one more thing, we assume that all of those stuff is reversible!
    What if the state of matter-energy at the very early stage was different than the matter is now days? What if we will never be able to figure out that?

  • The reason gravitation is so uniform is because it is an energy medium of infinite wave speed which ties all universe objects.

University of Vienna

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