I Why does particle physics predict cosmo constant is 10^120 too big?

[swampwiz]

I was reading this article at Wikipedia that says particle physics predicts that the cosmological constant is 10^120 larger than per observation:

https://en.wikipedia.org/wiki/Anthropic_principle

Steven Weinberg[11] gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude smaller than the value particle physics predicts (this has been described as the "worst prediction in physics").

 

[anuttarasammyak]

The cosmological constant means the absolute value of the energy density of the vacuum. In modern particle physics, it is theoretically known that the energy density of a vacuum is 10^56 to 10^120 times higher than the observed amount. Particle physics people regard vacuum as sea of particles generated and destroyed in short time. The discrepancy of astronomy estimation 10^-30 g/cc and particle physics theory 10^+26 g/cc are not filled out yet.

 

[PeterDonis]

it is theoretically known

No, it is theoretically predicted. This theoretical prediction is 120 orders of magnitude different from reality. We don't use the word "known" for such predictions.

 

[Ibix]

The problem is that particle physics (tested by measurement of the Casimir effect) predicts a certain energy density of the vacuum. It's tempting to assume that this is the same thing as the cosmological constant, which is a kind of background energy of spacetime in general relativity - but we can measure the effect of that on the orbits of planets and it's much smaller. So there's something we don't understand somewhere...

 

[Keith_McClary]

tested by measurement of the Casimir effect

But the Casimir effect can be explained by van der Waals forces, without need for the Zero Point Energy.

 

[Ibix]

But the Casimir effect can be explained by van der Waals forces, without need for the Zero Point Energy.

If it can be done either way presumably there's some relationship between thrir strengths. That half of the question is not an area I'm particularly confident in.

 

[Bandersnatch]

According to Baez, the whole question is something of an apples and oranges situation:
https://math.ucr.edu/home/baez/vacuum.html

 

[Vanadium 50]

If it can be done either way presumably there's some relationship between thrir strengths.

The exact opposite! If you can do the calculation without reference to the vacuum. the physics cannot depend on the vacuum!

The 10¹²⁰ number is not the result of any calculation. That's important, so let me say it again. The 10¹²⁰ number is not the result of any calculation.

It's a guess, based on "the only scale in the problem". To give an idea of how bad that reasoning is, suppose you wanted to know the distance between Earth and Mars. You know a Martian meteorite was found in Los Angeles, and you're 300 miles from Los Angeles. Since 300 miles is "the only scale in the problem" obviously the Earth-Mars distance must be around 300 miles. OK, maybe it's 30 and maybe 3000, but somewhere around 300 for sure.

 

[haushofer]

Interestingly, the famous "calculation" with the usual cut-off shows a UV divergence in an IR-phenomenon (i.e. large scales).

I agree that this is not really a "prediction". It's guessing at best.

 

[phyzguy]

Section IV of this paper goes into this question in some detail, emphasizing what @Vanadium 50 has said, that this cannot really be considered to be a "calculation"

 

[mathman]

http://ned.ipac.caltech.edu/level5/Weinberg/frames.html

Section 3 of this article gives a possible answer.

 

[phyzguy]

http://ned.ipac.caltech.edu/level5/Weinberg/frames.html

Section 3 of this article gives a possible answer.

Anthropic considerations might answer why the cosmological constant is so small, but I don't think they address the OP's question about the disparity with the particle physics "predeiction".

 

[kimbyd]

I was reading this article at Wikipedia that says particle physics predicts that the cosmological constant is 10^120 larger than per observation:

https://en.wikipedia.org/wiki/Anthropic_principle

Nobody knows how to calculate the cosmological constant from theory. Not yet. People have come up with various methods of doing so, but so far none are compelling. The simplest such way of calculating the cosmological constant is what leads to this 10^120 error, and it amounts to little more than dimensional analysis.

The way you should read this is that the value for the cosmological constant is very, very weird. Physicists generally feel that it is a number that demands an explanation outside of currently-known physics. Sadly, there aren't yet any clues as to where that answer might come from.

One answer that may be rather unsatisfying is that it may be a consequence of string theory and the weak anthropic principle. String theory, with its 10^400 or so distinct vacuum states, should easily permit numbers as small as this. But it should still be absurdly unlikely to get a number as small as 10^-120. However, if there was a selection effect which required the cosmological constant to be small, then maybe that could get us the rest of the way.

One such selection effect is the weak anthropic principle: observers can only observe conditions that allow them to exist. This principle can be used to explain why we find ourselves living on a planet that is very hospitable to life. Most planets we have observed are extremely inhospitable: no atmosphere, crushing atmosphere, freezing temperatures, blazing hot temperatures, etc. But we observe ourselves in a place that is really excellent for us. Why? Because organisms like ourselves could never have evolved in any of those inhospitable places. Organisms that can ask the question can only ever find themselves in a rather hospitable place (within reason).

And a large cosmological constant makes the universe very inhospitable indeed. If the value of the cosmological constant were only something like 1-2 orders of magnitude larger, no galaxies would ever have formed. So the answer to the question as to why the measured value is so low when the simplest theoretical prediction is so high might just be, "It actually takes a lot of different values, but we had to observe ourselves in a region with a very tiny value or else we couldn't exist in that region."

This answer is very unsatisfying to many theorists, who continue to feel that there must be another explanation. My suspicion is that the debate is unlikely to be settled anytime soon. It's hard to settle a debate when the evidence is scant.

 

[timmdeeg]

Nobody knows how to calculate the cosmological constant from theory. Not yet. People have come up with various methods of doing so, but so far none are compelling. The simplest such way of calculating the cosmological constant is what leads to this 10^120 error, and it amounts to little more than dimensional analysis.

Can we at least say the cosmological constant is an intrinsic property of vacuum energy? And if true wouldn't this mean that once we understand the true nature of vacuum the value of the cosmological constant isn't arbitrary but can in principle be derived from foundational principles.

During inflation the de Sitter like expansion of the universe was driven by a hypothetical scalar field. Could this be a hint that a tiny scalar field is the natural property of true vacuum?