We can’t know if the universe had a beginning

Leading cosmologist and Stephen Hawking co-author, George Ellis, in an interview about the frontiers of cosmology, and why we can never know if the universe had a beginning or existed forever.

Most people today believe in the theory of the big bang when it comes to the origin of the cosmos. Can we be sure that the universe had a beginning?

The history of the universe includes several stages. In very early times it went through an extraordinarily rapid period of accelerated expansion when it became enormously larger in a very short time; this is called inflation. At the end of inflation, that expansion had diluted all matter and radiation to nearly zero, but then the field that had caused inflation decayed into very hot matter and radiation, which continued to expand, but at a slower rate; that was the beginning of what we call the Hot Big Bang era. The physical processes that took place during this era are well understood and all cosmologists agree on what happened then.

What we don’t know is what happened before inflation started. The universe may or may not have had a beginning in that pre-inflationary era. The singularity theorems developed by Stephen Hawking do not apply, as it is now known that the required energy conditions were not met in those pre-inflationary times. In any case, a theory of quantum gravity is expected to apply at an early enough time, but we don’t know what that theory is. In summary, we don’t know if the universe had a beginning, but we do know that there was a Hot Big Bang.

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In either case, the universe would have existed for an infinite amount of time. That is indeed problematic because we could never prove that: we have no relevant observations to verify this.

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Is the inflation hypothesis on solid ground, or are there reasons to question it?

It stands on fairly solid ground and has one big plus: it offers a theory for the origin of primordial fluctuations that will later evolve into galaxies due to gravitational instability. We have no other theory that does that, and that is the main reason why it is accepted by most cosmologists.

The downside is that (a) we don’t have a solid theoretically grounded candidate for the inflaton – the field that causes inflation – that also gives the correct observation results, so basically it doesn’t have a solid link to fundamental physics. And (b) there’s a problem that’s been largely ignored, but which I think is important: how did the supposed quantum fluctuations that led to structure formation become classical? Most people ignore this issue, but I think it’s an important question.

If the universe had no beginning, that would mean that the universe has existed forever – for an infinite amount of time. But you said before that any theory that talks about infinity is not really a scientific theory because there is no way to prove that infinity exists. So if the universe has existed for an infinite amount of time, where would that leave the scientific status of cosmology?

If the universe had no beginning, it could have existed forever at an expansion rate that slowed down as we go back in time, but never reached zero, or it could have collapsed from a very large radius and then flipped over. In either case, the universe would have existed for an infinite amount of time. That is indeed problematic because we could never prove that: we have no relevant observations to verify this. However, it may have emerged from a very early era of a presently unknown time, when space and time did not exist. None of these possibilities affect the status of cosmology as a solid science for the study of all time from the start of inflation. This would just be another limit on what cosmology can determine, in addition to the limit already imposed by our visual horizons: the limit on how far back, in the history of the universe, we can see matter (that is, when matter and radiation disconnected as the universe cooled and became transparent). Every scientific theory has limits to its applicability, and so do our cosmological models. It is a good model within its scope.

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The main problem for cosmology is that there is only one universe. That makes it different from all other sciences. We cannot repeat the universe and see what happens; we can’t compare it to other universes

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One of the presuppositions of cosmology early on was the so-called Copernican assumption that the universe is the same everywhere and obeys the same natural laws. Can we test whether that is the case, and how can we tell the difference between recalcitrant observations of distant regions of space that signal our theories to be revised, and those regions are in fact governed by different laws of nature?

This is an area where great progress has been made in recent decades: there are now a number of perceptual tests of the Copernican principle within our visual horizon. intriguing, a recent article suggests that there could be a problem in this regard, something that would challenge the Standard Model of cosmology. But the fact that the Copernican principle can be challenged by observational data shows that it is a testable principle!

However, there is no indication that the laws of physics are different anywhere in the universe than here: indeed the spectrum of the relic Cosmic Background Radiation left over from the Hot Big Bang era has an exact Black Body spectrum, as determined by Planck more than one century ago, to within the perceptual limits of the spectrum. This proves that both quantum physics and statistical physics were the same then as they are here and now. Observations of extremely distant galaxies and quasars point to the same thing. The laws of nature seem reliable everywhere.

You wrote before that our cosmological models are not determined by the data we have at our disposal. What do you mean by that, and is this a cosmology-specific problem, or, as some philosophers of science have argued, something that applies to all scientific theories?

The main problem for cosmology is that there is only one universe. That makes it different from all other sciences. We cannot repeat the universe and see what happens; we cannot compare it to other universes; we are stuck in our own galaxy and cannot go to another point of view to see what the universe looks like from there, because of its sheer scale. All we have to work with is an image of what is beyond, at all distances, as seen on a two-dimensional sphere (“the sky”). Our problem is determining how far each of the objects we see is. And the point is that we see the more distant areas sooner than the near ones, because of the enormous time it takes to reach the light from there. So the circumstances were different then. How do we know if we see a certain size or brightness because they are at a certain distance, or rather because their properties were different then? For example, different metals in the environment can alter the supernova’s brightness curves. This problem is unique to cosmology.

What is the biggest crack in the standard cosmological model as it stands that could eventually topple it?

There are two main issues: the potential anisotropy issue discussed in the article linked above, and the issue that the values determined for the universe’s expansion rate — the Hubble constant — seem to differ as we take it from more locally or estimate further away. observations. Both may point to the need for a more complex cosmological model than the Standard Model – one with anisotropy or inhomogeneity, unlike the Standard Model.

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Ultimately, the question of why the universe has specific initial conditions is not scientific. It’s a metaphysical issue with several options.

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There are increasing voices claiming that in the absence of direct evidence for the existence of dark matter and dark energy, we should leave the current cosmological model behind and adopt what is known as MOND, a Modified Newtonian Dynamics model. . What do you think of this argument?

It is a serious proposal that should be carefully considered. There are problems because it is a Newtonian type model, but there have been careful analyzes suggesting that it could be correct. But MOND deserves further research and needs to be fully developed into a model comparable to Einstein’s general theory of relativity.

One of the things that has puzzled cosmologists is why the universe seems to be just right in terms of different cosmological constants for the development of life. What do you think best explains this apparent fine-tuning of the universe?

Well, the standard scientific explanation is that we live in a multiverse with countless expanding universe domains like the one we live in, but with different physics in each of them; in that case, it’s going to be okay that there’s life in some of those bubbles, just by chance, so it becomes likely anyway.

I’m skeptical because it’s not an observation-testable hypothesis, it’s not clear what mechanism will result in different physics existing in each of these domains, if they exist, and in any case it just pushes the apparent fine-tuning up a level : Why is the multiverse tuned to be just right for life? The same problem arises at that level.

Ultimately, the question of why the universe has specific initial conditions is not scientific. It’s a metaphysical issue with several options. I’ll leave it at that.

You’ve also talked about the idea that the universe is evolving. What do you mean? Does this go beyond saying that the universe is changing?

The evolution of the universe is nothing like evolution in the case of organisms and natural selection. The term simply means that the properties of the universe – its size (if it has a positive spatial curvature), expansion rate, density, temperature, and so on – change over time, in a way that is open to scientific investigation. It’s just like you can speak of an oak evolving as it grows from an acorn to a majestic fully developed specimen. So yeah, it’s just saying the universe is changing.

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