There is no singularity established within the formal theory itself. The BBT, being scientific, only goes to where the physics equations run true. Just before and at t=0, the physics falls completely apart. Lemaitre envisioned his model to get down to something like the size of an atom, which he called the Primeval atom. He could have claimed a singularity since the very first solution to GR, outside of Einstein, came from Schwarzschild work with black holes, etc. But Lemaitre wisely avoided doing so.Helio, I don't think we finished the discussion on the origin of the BB, viz, backward extension of an expansion graph to a singularity?
Yes, a tiny glob for a recently crunched universe might be identical to what we claim for the tiny glob just after t=0. But, if so, the crunched version would need to be extremely isotropic and homogenous within that glob and I don't know if Humpty Dumpty would look identical if we throw his parts back into one spot his original size, if you see my point. It's just over my head but it is a key question to ask. Entropy would be next on my list, but even if this limits the number of cycles, then only more than one is needed.Even so, I don't believe one could tell the difference between a singularity BB event, and a cyclic Universe where the singularity is replaced by a nexus. At t = 0, within a whisker, physics breaks down. Without the singularity, physics need not break down. I can't see how any of this is at variance with your bullets, but please correct me if I am in error.
Yes.In BBT, do you envisage the singularity, or the first 'object' before inflation begins as isotropic and homogenous?
I would expect any tiny variation in energy distribution initially would be greatly expanded and easily observable in our universe. I don't mean perfect isotropy because quantum levels are never purely isotropic. Indeed, one of the first problems with BBT came when quantum physics recognized this fact and could not explain the great isotropy (relatively speaking) of what we observe, especially in the CMBR.And, if so, why?
Spock may have said it best when he described the first instant as having "Pure energy", as if there's such a thing as impure energy. The temperatures near the first Planck time unit was in the many billions of trillions of degrees, IIRC. But, again, I am not going beyond physics and assuming a singularity, which is speculation and not part of a theory that requires tests.I don't understand what that object might be like?
There is a point where physics works reasonably and reliably well, but a tiny bit farther, the physics' equations produce results that shoot to infinity. So that also is the line between science and philosophy, though perhaps "metaphysics" might be the apt term.Since we cannot go back and examine it, are we talking philosophy and not science anyway? In which case, I cannot see any of this being counter to nexus over singularity.
I hope this is not for me since you know I have no interest in even suggesting a singularity as a starting point. But you may be bringing-up an interesting question because Hawking demonstrated that the area of the EH around the singularity (BH) gives the entropy of the BH. So, if that is a physical reality, imagine the huge size of the EH radius, yet no space to have it. A paradox?Is entropy at minimum in a singularity at infinite density/pressure/temperature?
Entropy comes from processes, so they began at the beginning but I understand the entropy at the beginning was amazingly small, or negative if that helps. Max. entropy would just be very low level heat. The entropy of your refrigerator, for instance, dumps heat into the universe in order to provide a little less negative entropy for cooling. Entropy isn't just high temperature (as seen near t=0) but heat exchange along an isotherm. I've never gotten my head around entropy but I recall using it a lot to pass thermodynamics.Surely entropy must increase after the BB?
I doubt many cosmologist would favor this CMBR temperature interpretation. Clouds can be misleading. The coldest known place in the universe isn't empty space but places like the Boomerang Nebulae at ~ 1K.Here is an example. Shadow of cosmic water cloud reveals the temperature of the young universe, https://phys.org/news/2022-02-shadow-cosmic-cloud-reveals-temperature.html, Feb-2022. My note. This new report for HFLS3 suggest most studies fall z = 0 to 1.0 range with a smaller number 1.8 to 3.3, now this report z=6.34 when plotting CMB temperature cooling to redshift values. The CMB temperature value for z=6.34 was found to be 16.4 - 30.2 K TCMB. This is warmer than the 3K we see on Earth but is a long way from extending out 45 or 46 billion light years and arriving at 3000 K or so and z~1100.
Yes. A lot of questions are indeed being asked. The idea of the entire universe of what we can see and what we think is there squeezed into something like the size of an atom will not be something Newton could have handled in his day had he speculated about it. The very earliest history of BBT, and worse before it, involves only extreme quantum theory and extreme GR. The trick is to combine the two but they don't even like each other.Some questions could be asked. What is the vacuum energy density of the universe before the postulated inflation epoch? Do we see in astronomy today space with this vacuum energy density today? My answer is no unless someone argues about black holes at their center perhaps (but I do not see the vacuum energy density at the bottom of a black hole).
Alan's work is like the Wright Bros. aeroplane -- a wonder then but not that practical today. Any temperature had to be based on assumptions, especially how much expansion has taken place since Recombination. Had he nailed it and done more publicizing it, perhaps, then he may have gotten more credit. It's my understanding he had to fight to get people to recognize his early efforts.How fast was 4D space expanding during inflation epoch to solve the horizon problem that would be seen in the CMBR today (again Alan Guth mentions)? My answer is Alan Guth shows at least 10^20 c or faster. Do we see in astronomy today 4D space expanding 10^20 c or faster? My answer is no. Some of this post touches upon post #8 and where are the magnetic monopoles that we could see today. What I like to call exotic physics, need rigorous examination and evidence shown like the laws of motion.
Do you see this as contradictory to mainstream cosmologies' estimates of today's CMBR? [It may be a while before I have time to read some of your links.]Helio, your post #11 I concur, *unclear what they are saying*. I had to eat that report and kept track of others published over the years before the TCMB compared to redshift plots started to make sense. Example.
A precise and accurate determination of the cosmic microwave background temperature at z = 0.89, https://ui.adsabs.harvard.edu/abs/2013A&A...551A.109M/abstract, March 2013. "...Results: We determine TCMB = 5.08 ± 0.10 K at 68% confidence level. Our measurement is consistent with the value TCMB = 5.14 K predicted by the standard cosmological model with adiabatic expansion of the Universe. This is the most precise determination of TCMB at z > 0 to date."
The original 3000K temperature is what we would have seen 13.8 billion years ago because everything in the universe was in this sea of light. With expansion, the light that was farther away from us but headed in our direction eventually reached us, but this was, of course, a continuous stream of light. This light that has finally reached has redshifted (z = 1100) and is now in the microwave band. Thus, with greater redshift, the temperature of what we observe will continue to be less and less.What we do not see is z = 1100 and TCMB 3000 K. There is a limited number of redshift plots for TCMB changes documented and these are lower number redshifts, not objects with z like 50, 100, 500, etc. As the redshifts get larger and larger, the CMBR temperature should get warmer and warmer and follow the BB model cooling curve for expanding 4D space, otherwise Houston, we have a problem
Cat interesting questions. I do not know the answers but I noticed from the link you provide (courses.lumenlearning) it says, “But in terms of the universe, and the very long-term, very large-scale picture, the entropy of the universe is increasing, and so the availability of energy to do work is constantly decreasing. Eventually, when all stars have died, all forms of potential energy have been utilized, and all temperatures have equalized (depending on the mass of the universe, either at a very high temperature following a universal contraction, or a very low one, just before all activity ceases) there will be no possibility of doing work. Either way, the universe is destined for thermodynamic equilibrium—maximum entropy. This is often called the heat death of the universe, and will mean the end of all activity. However, whether the universe contracts and heats up, or continues to expand and cools down, the end is not near. Calculations of black holes suggest that entropy can easily continue for at least 10^100 years.”Can I please have your answers/suggestions on one question, which afaik, has been touched on, but not answered/explained to my satisfaction.
What is the entropy of the Universe at or slightly after t = 0?
Or does the concept of entropy at t = 0 break with the rest of physics?
What is the entropy when physics becomes operative?
I can see arguments for being minimum or maximum.
Also, is a cyclic Universe (if it exists - hence metaphysical model like the BB) , a closed system? Hence (also metaphysical) would entropy decrease during a contracting phase between one nexus and the next? Assuming contraction follows expansion.
https://courses.lumenlearning.com/physics/chapter/15-6-entropy-and-the-second-law-of-thermodynamics-disorder-and-the-unavailability-of-energy/#:~:text=Entropy is the loss of,increases in an irreversible process.