The Deep Infinite Universe Puzzle |  George Ellis

The Deep Infinite Universe Puzzle | George Ellis

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

Most people today believe in the Big Bang theory when it comes to the origins of the cosmos. Can we be certain that the universe had a beginning?

The history of the universe includes several stages. In the very beginning, it went through a period of extraordinarily rapid accelerated expansion when it grew enormously larger in a very short time; this is called inflation. When the inflation ended, this expansion had caused all matter and radiation to dilute to almost zero, but then the field that had caused the inflation had broken down into very hot matter and radiation that continued to dilate, but at a slower rate; this was the start of what we call the Hot Big Bang Era. The physical processes that occurred at 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 this pre-inflationary era. The singularity theorems developed by Stephen Hawking do not apply, because it is now known that the required energy conditions are not satisfied at this pre-inflationary moment. Either way, a theory of quantum gravity should apply soon enough, but we don’t know what that theory is. To sum up: 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 both cases, the universe would have existed for an infinite time. This is indeed problematic because we have never been able to prove it: we have no relevant observations to verify it.

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Is the inflation assumption sound or are there reasons to question it?

It is on reasonably solid ground and has one great advantage: it provides a theory for the origin of primordial fluctuations that will later turn into galaxies through gravitational instability. We don’t have any other theory that does this, and that’s the main reason it’s accepted by most cosmologists.

The downside is that (a) we don’t have a theoretically strong candidate for the inflaton – the field causing inflation – which also gives the right observational results, so in fact it’s unrelated solid with fundamental physics. And (b) there is a question mostly overlooked but which I believe is important: how did the supposed quantum fluctuations which led to the formation of the structure become classical? Most people ignore this problem, but I think it’s an important question.




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If the universe had no beginning, that would presumably mean that the universe has existed forever – for an infinite duration. 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 there is an infinity of anything. So if the universe has been around 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 with a slowing rate of expansion as we go back in time but never reaching zero, or it could have collapsed from ‘a very large radius then turn around. In both cases, the universe would have existed for an infinite time. This is indeed problematic because we have never been able to prove it: we have no relevant observations to verify it. He could, however, have emerged from a very ancient time of a nature unknown at the present time, where 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 since inflation began. It would simply be another limit to what cosmology can determine, in addition to the limit already imposed by our visual horizon: the limit from which, in the history of the universe, we can see matter (that’s that is, when matter and radiation decoupled from each other as the universe cooled and became transparent). Any scientific theory has limits to its applicability, and so do our cosmological models. It is a good model in its field of application.

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The key problem for cosmology is that there is only one universe. This differentiates it from all other sciences. We can’t rerun the Universe and see what happens; we cannot compare it with other universes

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One of the presuppositions of cosmology from the very beginning was the so-called Copernican hypothesis that the universe is the same everywhere and obeys the same laws of nature. Can we test if this is the case, and how could we differentiate between recalcitrant observations of distant regions of space that indicate our theories need to be revised and those regions that are in fact governed by different laws of nature ?

This is an area where great progress has been made over the past few decades: there are now a number of observational tests of the Copernican Principle within our visual horizon. Curiously, a recent paper suggests there might be a problem with this, something that challenges 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 any different anywhere in the universe than they are here: indeed, the relic spectrum of cosmic background radiation left over from the Age of Hot Big Bang has an exact black body spectrum, as determined by Planck over a century ago, within the limits of spectrum observation, This proves that quantum physics and statistical physics were the same back then that they are here and now. Observations of extremely distant galaxies and quasars indicate the same. The laws of nature seem reliable everywhere.


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You wrote earlier that our cosmological models are indeterminate by the data we have. What do you mean by that, and is it a problem specific to cosmology, or, as some philosophers of science have argued, something that is true for all scientific theories?

The key problem for cosmology is that there is only one universe. This differentiates it from all other sciences. We can’t rerun the Universe and see what happens; we cannot compare it with other universes; we are stuck in our own Galaxy and cannot go to another vantage point to see what the Universe looks like because of its vast scale. All we have to work with is an image of what is out there, at all distances, as seen on a 2-dimensional sphere (“the Sky”). Our problem is to determine how far away each of the objects we see is. And the thing is, we see the farther ones sooner than the near ones, because of the huge amount of time it took for the light to come in from there. The conditions were therefore different at the time. How do we know if we see a certain size or brightness because they are a certain distance away, or rather because their properties were different at that time? For example, different metals in the environment could alter the brightness curves of supernovae. This problem is peculiar to cosmology.

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

There are two key problems: the problem of possible anisotropy discussed in the article linked above, and the problem that the values ​​determined for the rate of expansion of the Universe – the Hubble constant – seem to differ depending on whether we estimate it from more local or more distant data. observations. Either could indicate 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 unscientific. It is a metaphysical question with various options.

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There are a growing number of voices arguing that in the absence of direct evidence for the existence of dark matter and dark energy, we should abandon the current cosmological model and embrace what is known as MOND – a model of modified Newtonian dynamics. What do you think of this argument?

This is a serious proposition which must be carefully considered. There are problems in that it is a Newtonian-like model, but careful analyzes suggest it might be correct. But MOND deserves further investigation and needs to be fully developed into a model similar to Einstein’s theory of general relativity.


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One of the things that has puzzled cosmologists is why the universe seems to be fair in terms of various 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 millions of expanding universe domains like the one we live in, but with different physics in each one; in this case, things will work out for life to exist in a few of these bubbles just by chance, so it becomes likely after all.

I’m skeptical about this because it’s not an observationally testable hypothesis, it’s unclear what mechanism will result in different physics existing in each of these domains, if they exist, and in any case, it doesn’t only takes the seeming fine-tuning up a level: why is the multiverse tuned to be perfect for life? The same problem occurs at this level.

Ultimately, the question of why the universe has specific initial conditions is unscientific. It is a metaphysical question with various options. I will leave it there.

You also talked about the idea of ​​the evolution of the universe. What do you mean? Does it go beyond saying that the universe is changing?

The evolution of the universe has nothing to do with 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 space curvature), rate of expansion, density, temperature, etc. – change over time, in a way open to scientific investigation. It’s like how you can talk about an oak tree evolving as it grows from an acorn to a fully grown majestic specimen. So yeah, that’s just saying the universe is changing.


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