Big Bang Notes II: Ethan Siegel

Picture from Did the Big Bang begin from a singularity?

After the notes I made in my last post on Big Bang Theory I was  fascinated by the following article on "The Big Think" by  Big Bang theorist Ethan Siegel.....

Did the Big Bang begin from a singularity? Not anymore. - Big Think

Below I quote parts of the article and interleave my comments. 

SIEGEL: But extrapolating beyond the limits of your measurable evidence is a dangerous, albeit tempting, game to play. After all, if we can trace the hot Big Bang back some 13.8 billion years, all the way to when the universe was less than 1 second old, what’s the harm in going all the way back just one additional second: to the singularity predicted to exist when the universe was 0 seconds old?

The answer, surprisingly, is that there’s a tremendous amount of harm — if you’re like me in considering “making unfounded, incorrect assumptions about reality” to be harmful. The reason this is problematic is because beginning at a singularity — at arbitrarily high temperatures, arbitrarily high densities, and arbitrarily small volumes — will have consequences for our universe that aren’t necessarily supported by observations.

 For example, if the universe began from a singularity, then it must have sprung into existence with exactly the right balance of “stuff” in it — matter and energy combined — to precisely balance the expansion rate. If there were just a tiny bit more matter, the initially expanding universe would have already recollapsed by now. And if there were a tiny bit less, things would have expanded so quickly that the universe would be much larger than it is today.

And yet, instead, what we’re observing is that the universe’s initial expansion rate and the total amount of matter and energy within it balance as perfectly as we can measure.

 Why?

 If the Big Bang began from a singularity, we have no explanation; we simply have to assert “the universe was born this way,” or, as physicists ignorant of Lady Gaga call it, “initial conditions.”

 Similarly, a universe that reached arbitrarily high temperatures would be expected to possess leftover high-energy relics, like magnetic monopoles, but we don’t observe any. The universe would also be expected to be different temperatures in regions that are causally disconnected from one another — i.e., are in opposite directions in space at our observational limits — and yet the universe is observed to have equal temperatures everywhere to 99.99%+ precision.

 We’re always free to appeal to initial conditions as the explanation for anything, and say, “well, the universe was born this way, and that’s that.” But we’re always far more interested, as scientists, if we can come up with an explanation for the properties we observe.

MY COMMENT: Siegel is warning us against the extrapolating right back to a space-time singularity. That makes sense to me on this basis: I always have doubts when a theory predicts an infinity and I'm inclined to believe that this is a sign of an incomplete theory that is being pushed too far.  But in the above quote Siegel's reason for rejecting an initial singularity is to do with scientific prediction: For unless one is to engage in the ad hoc business of patching in arbitrary initial conditions, a cosmos that starts with an arbitrarily high temperature doesn't perform well on the prediction front. Siegel then goes on to tell us that a good origins theory would predict important cosmic features like the  flatness of space, the absence of magnetic monopoles, and the uniformity of temperature and density across the observable universe.  

As we shall see Siegel doesn't contradict Einstein's great theory of gravitation which predicts the possibility of a space-time singularity. Instead he conveniently side steps the question of whether space time space-time singularities are physical by telling us to stop yourself before you go all the way back to a singularity.

SIEGEL: Inflation accomplishes [(correct) predictions] by postulating a period, prior to the hot Big Bang, where the universe was dominated by a large cosmological constant (or something that behaves similarly): : the same solution found by de Sitter way back in 1917. This phase stretches the universe flat, gives it the same properties everywhere, gets rid of any pre-existing high-energy relics, and prevents us from generating new ones by capping the maximum temperature reached after inflation ends and the hot Big Bang ensues. Furthermore, by assuming there were quantum fluctuations generated and stretched across the universe during inflation, it makes new predictions for what types of imperfections the universe would begin with.

 MY COMMENT: So, inflation theory predicts a) a near enough flat universe, b) the absence of high energy relics (like magnetic monopoles), c) essentially a uniform distribution and d) makes predictions about the magnitude of fluctuations away from perfect uniformity.  Sounds good so far.  Now comes the interesting bit:

SIEGEL: But things get really interesting if we look back at our idea of “the beginning.” Whereas a universe with matter and/or radiation — what we get with the hot Big Bang — can always be extrapolated back to a singularity, an inflationary universe cannot. Due to its exponential nature, even if you run the clock back an infinite amount of time, space will of time, space will only approach infinitesimal sizes and infinite temperatures and densities; it will never reach it. This means, rather than inevitably leading to a singularity, inflation absolutely cannot get you to one by itself. The idea that “the universe began from a singularity, and that’s what the Big Bang was,” needed to be jettisoned the moment we recognized that an inflationary phase preceded the hot, dense, and matter-and-radiation-filled one we inhabit today.

This new picture gives us three important pieces of information about the beginning of the universe that run counter to the traditional story that most of us learned. First, the original notion of the hot Big Bang, where the universe emerged from an infinitely hot, dense, and small singularity — and has been expanding and cooling, full of matter and radiation ever since — is incorrect. The picture is still largely correct, but there’s a cutoff to how far back in time we can extrapolate it.

MY COMMENT: Yes, I get the point: Running a positive exponent exponential backwards means that it never reaches that mathematically mysterious singularity.  And yes we may well need to jettison the singularity postulate. I for one regard it as ontologically suspicious and unlikely to be physical. 

SIEGEL:  Lastly, and perhaps most importantly, we can no longer speak with any sort of knowledge or confidence as to how — or even whether — the universe itself began. By the very nature of inflation, it wipes out any information that came before the final few moments: where it ended and gave rise to our hot Big Bang. Inflation could have gone on for an eternity, it could have been preceded by some other nonsingular phase, or it could have been preceded by a phase that did emerge from a singularity. Until the day comes where we discover how to extract more information from the universe than presently seems possible, we have no choice but to face our ignorance. The Big Bang still happened a very long time ago, but it wasn’t the beginning we once supposed it to be.

MY COMMENT: Yes, I can accept Siegel's talk about our ignorance: in fact Siegel himself doesn't comment on two outstanding questions: Viz: What provides the energy for inflation? The nearest he gets to this question is a reference to a the cosmological constant which is another patch-in not greatly different to patching in initial conditions to fix the problems. The other baffling issue is this: As we follow the shrinking exponential of inflation back in time there comes a point where the scales of gravity and quantum theory collide: What happens then?   But quoting Siegel once more we've at least got this to hang onto:

The [Big Bang] picture is still largely correct, but there’s a cutoff to how far back in time we can extrapolate it.

So further extrapolation beyond the hot big bang period is an extrapolation into the dark unknown. Therefore, apart from speculation on all sides, I guess that is how the situation will remain for some time to come. As I said in my last post on Big Bang: People still hanker and yearn after the idea that there was something  before the big bang. But what was it? Was it God or just more  algorithmically compressible bytes and bits?  It might help when the incommensurability of gravitational theory and quantum theory is sorted.