by Distinguished cosmologist and co-author of Stephen Hawking, George Ellis, in an interview about the limits of cosmology and why we may never know whether the universe had a beginning or has existed forever.
Most people today believe in the Big Bang theory when it comes to the origin of the Universe. 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 extremely rapid period of accelerated expansion when it became much larger in a very short time. this is called inflation. At the end of inflation, this expansion had caused all matter and radiation to dilute to almost zero, but then the field that had caused inflation decayed into very hot matter and radiation that continued to expand, but at a slower rate. This was the beginning of what we call the Hot Big Bang Era. The physical processes that occurred during this time 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 hold because the required energy conditions are now known not to be satisfied in that pre-inflationary period. In any case, a theory of quantum gravity is expected to be implemented quite early, but we don’t know what that theory is. To summarize: 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 exist for infinite time. This is indeed problematic because we have never been able to prove it: we have no relevant observations to check it.
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Is the inflation hypothesis on solid ground, or are there reasons to question it?
It is on pretty solid ground and has one big advantage: it provides a theory for the origin of the primordial fluctuations that will later evolve into galaxies due to gravitational instability. We have no other theory that does this, and this is the main reason it is accepted by most cosmologists.
The downside is that (a) we don’t have a strong theoretically grounded candidate for the inflaton—the field that causes inflation—that also gives the right observational results, so it doesn’t really have a firm connection to fundamental physics. And (b) there is an issue that is usually ignored, but one that I think is important: how did the supposed quantum fluctuations that led to the formation of the structure become classical? Most people ignore this topic, but I think it’s an important question.
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If the universe had no beginning, this would probably mean that the universe has existed forever – for an infinite amount of time. But you’ve said before that any theory that talks about infinity isn’t really a scientific theory, since there’s no way to prove that there’s an infinity of anything. So if the universe exists for an infinite amount of time, where would that leave the scientific regime of cosmology?
If the universe had no beginning, it could have existed forever with an expansion rate getting slower as we go back in time but never reaching zero, or it could have collapsed from a very large radius and then turned. In either case, the universe would exist for infinite time. This is indeed problematic because we have never been able to prove it: we have no relevant observations to check it. It could, however, have arisen from a very early epoch of an as yet unknown nature, when space and time did not exist. Neither of these possibilities affects cosmology’s status as a stable science for the study of all time since the beginning of inflation. This would just be another limit on what cosmology can determine, in addition to the limit already imposed by our visual horizon: the limit on how far back in the history of the universe we can see matter (that is, when matter and radiation decoupled from each other as the universe cooled and became transparent). Any scientific theory has limits to its application, and so do our cosmological models. It is a good model in its scope.
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The key issue for cosmology is that there is only one Universe. This makes it different from all other sciences. We can’t run the Universe again and see what happens. we cannot compare it with other universes
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One of the early premises of cosmology was the so-called Copernican hypothesis that the universe is the same everywhere and obeys the same laws of nature. Can we test whether this is the case and how might we distinguish between spurious observations of distant regions of space that indicate our theories need revision versus those regions that are actually governed by different laws of nature?
This is an area where much progress has been made in recent decades: there are now a number of observational tests of the Copernican principle on our visual horizon. Curiously, a recent paper suggests that there might be a problem in this regard, which 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 evidence that the laws of physics are any different anywhere in the universe than they are here: indeed the spectrum of cosmic background radiation left over from the Hot Big Bang era has an exact Blackbody spectrum, as determined by Planck over a century ago, within the observational 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 show the same thing. The laws of nature seem everywhere reliable.
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You have written before that our cosmological models are not determined by the data available to us. What do you mean by that, and is this a particular problem of cosmology or, as some philosophers of science have argued, something that applies to all scientific theories?
The key issue for cosmology is that there is only one Universe. This makes it different from all other sciences. We can’t run the Universe again and see what happens. We cannot compare it with other universes. We are stuck in our own Galaxy and can’t go to any other vantage point 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’s out there, at all distances, as seen on a two-dimensional sphere (“the sky”). Our problem is to determine how far each of the objects we see is. And the point is that we see the more distant ones at earlier times than the closer ones, because of the huge amount of time it took for the light to get there. Conditions 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 at the time? For example, different metals in the environment can change the light curves of supernovae. This issue is unique to cosmology.
What is the biggest crack in the cosmological standard model as it stands today that could end up overturning it?
There are two main issues: the possible anisotropy issue discussed in the paper cited above, and the problem that the values determined for the expansion rate of the Universe – the Hubble constant – seem to differ depending on whether we calculate it from a more local or more distant observations. Either may indicate the need for a more complex cosmological model than the standard model – one with anisotropy, or inhomogeneity, as opposed to the standard model.
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Ultimately, the question of why the universe has certain initial conditions is not a scientific one. It is a metaphysical theme with various options.
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There are more and more voices arguing 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 proposition that should be carefully considered. There are problems with it being a Newtonian model, but there have been careful analyzes that suggest it may be correct. But MOND deserves further research and needs to be fully developed into a model similar to Einstein’s general theory of relativity.
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One of the things that has puzzled cosmologists is why the universe seems to be right about the various cosmological constants for the development of life. What do you think best explains this apparent perfection of the universe?
Well, the standard scientific explanation is that we live in a multiverse with billions of expanding universe sectors like the one we live in, but with different physics in each one. In that case things will go well for there to be life in some of these bubbles just by chance, so eventually it becomes possible.
I’m skeptical about this because it’s not a hypothetically testable hypothesis, it’s not clear what mechanism would lead to different physics in each of these domains, if any, and in any case, it just pushes the apparent detail up one level: because is the multiverse set up to be suitable for life? The same problem occurs at this level.
Ultimately, the question of why the universe has certain initial conditions is not a scientific one. It is a metaphysical theme with various options. I’ll leave it at that.
You have also talked about the idea of the evolution of the universe. What do you mean by that? Does this go beyond saying that the universe is changing?
The evolution of the universe is nothing like the evolution in the case of organisms and natural selection. The term simply means that the properties of the universe – its size (if it has positive spatial curvature), expansion rate, density, temperature, and so on – change over time in a way that is open to scientific investigation. It’s like the way you might talk about an oak tree evolving as it grows from an acorn to a majestic fully grown specimen. So yeah, it just says the universe is changing.
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