The latest BICEP experiment analysis, using data through 2018, further constrains the magnitude of any primordial gravitational waves (strictly speaking the "scalar to tensor ratio").
This further narrows the parameters space allowed for cosmological inflation theories and the data are consistent with the non-existence of primordial gravitational waves. It also continues to be consistent with a universe that isn't quite scale free.
The abstract of the preprint states:
The likelihood analysis yields the constraint
r0.05<0.036at 95% confidence.
Running maximum likelihood search on simulations we obtain unbiased results and find that σ(r)=0.009.
These are the strongest constraints to date on primordial gravitational waves.
The body text notes that:
The BKP analysis yielded a 95% confidence constraint r0.05 < 0.12, which BK14 improved to r0.05 < 0.09, and BK15 improved to r0.05 < 0.07. The BK18 result described in this letter, r0.05 < 0.036, represents a fractional improvement equivalent to the two previous steps combined. The BK18 simulations have a median 95% upper limit of r0.05 < 0.019.
The distributions of maximum likelihood r values in simulations where the true value of r is zero gave σ(r0.05) = 0.020 for BK15 which is reduced to σ(r0.05) = 0.009 for BK18. . . .
The system is projected to reach σ(r) ∼ 0.003 within five years with delensing in conjunction with SPT3G.
Previous coverage at this blog on October 6, 2020, July 24, 2017 and March 18, 2014. The chart from October 6, 2020 was as follows:
A short 2019 paper reviewed some of the cosmological inflation models that are still consistent with this data.
are there any good alternative to cosmological inflation models ?
There are significant disputes among specialist scientists over whether cosmological inflation is necessary at all, and if so, why. Cyclic and bouncing cosmologies tend not to need them either.
IMHO, we simply don't have good enough data and sound enough models to know, so it should be largely a back burner issue until we improve in both the observation department and the theory department.
Since you are discussing matters related to the ones in my comment here, let me state this:
Yes, back burner can be a good description. Until we have more solid data, we should refrain from making strong statements about the state of the universe in the early fractions of a second after the hypothesized moment of Big Bang, the Big Bang itself (whatever it was) and what, if anything, came before the Big Bang IMO. But we should not slow down in tackling these matters at the same time.
So speaking about a beginning and an age of the universe as if they are based on solid physical evidence is premature at the moment IMO.
The period of time that has elapsed from a Big Bang-like event to the present is based upon much more solid evidence from multiple sources, and is much more robust, than cosmological inflation, or even the cosmological constant/dark energy.
It isn't a terribly precise estimate (on the order of 1% precision), and it doesn't truly resolve the question of what, if anything, came before the Big Bang. But it is far less uncertain than most other issues in cosmology and is robust to a variety of alternative scenarios for the immediate time period around the Big Bang-like moment in the history of the observable Universe.
Did not mean to say the calculated period of time from what is commonly called the Big Bang (whatever it really was) to the present, which is roughly 13.8 billion years, is inaccurate, I already accept its accuracy. What I say is highly open to dispute is what happened in the early fractions of a second after the hypothesized moment of Big Bang, during the Big Bang itself and before, if there is, the Big Bang. That is why I say the age of the universe is far from certain as we do not know if the Big Bang is the beginning of the universe or whether there really is a beginning to the universe. The universe may well have no beginning if one of the cyclic models or some of the other cosmological models is accurate. What I simply propose is that currently we do not have enough evidence to declare any of the existing cosmological models the winner as we cannot currently observationally trace the universe to the moment of Big Bang or the critical early fractions of a second after the Big Bang.
Of course the facts I mention here are common knowledge among physicists. But these facts are rarely presented to the general public, the general public is in general presented a picture with the cosmological models that take the Big Bang event (whatever it was regardless of the naming) that occurred about 13.8 billion years ago (there were an infinite number of Big Bang events in the past of the universe according to the cyclic models at least) as the beginning of the universe, or of our universe if we accept the multiverse models, as competing among each other and with no other cosmological models taken seriously. So the cosmological models that take the Big Bang of 13.8 billion years ago as a universe beginning, whether they are single universe models or multiverse models, are presented to the general public as if they are the only viable alternatives. Physicists are partly responsible for this general public misconception. Physicists should make it clear to the general public that we cannot trace the universe back to the Big Bang event of 13.8 billion years ago or the critical early fractions of a second after that with the observation methods we currently have and thus cannot currently know whether that Big Bang event is the beginning of the universe (or our universe using a multiverse model terminology) and the universe (or this universe) has a past before that event or not. The general public rarely hears of the cyclic models for instance, most people are not even aware of them, this is a result of erroneous or deficient ways of presenting the cosmological data and facts to the general public.
To give readers a context on the cyclic cosmological models, on another thread of this blog I had written a series of comments on them beginning from this comment.
Like I pointed out on the thread I link to, cosmological models with a universe that has a beginning have to explain away the singularity at the beginning of the universe in order to physically make sense because singularities are physically impossible. Cyclic models do not have that problem since they do not have a universe with a beginning hence no initial singularity to explain away; also they usually do not need cosmological inflation and often better explain dark matter and dark energy phenomena.
This comment is only for Abrahamic theists, not for Andrew and other non-Abrahamic theists:
Many Abrahamic theists consider the hypothesis that the Big Bang is the beginning of the universe an essential part of their belief system and hence support that hypothesis for religious reasons (among others). But God as commonly conceived in the Abrahamic religions is beyond space, time and infinity and is the creator of space, time and infinity. He can theoretically create space and time (spacetime) as infinite in all directions (including the time directions) with his infinite power according to the common Abrahamic conception of God, there is nothing against this in the common Abrahamic theology. So cyclic cosmological models with their conception of the universe as without a beginning and an end and as infinite in all directions in spacetime do not contradict the common Abrahamic theology.
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