# Qualitative picture or reference for a Lemaître's Cold Big Bang theory

Warning: please, consider this question to be motivated by historical curiosity or as an exercise in model-building. I believe this cannot be considered non-mainstream physics as it was very much mainstream in the past.

Historical overviews of the Hot Big Bang theory usually concentrate on its steady state rival — the Fred Hoyle's theory where the Universe was expanding but essentially stayed the same due to creation of energy in space that locally balanced out the expansion.

But I often think about another and naively more plausible idea of Lemaître: the "primeval egg" or the Cold Big Bang theory. I've read that it was disproven my WMAP's discovery of BAO (baryon-acoustic oscillations), but this strikes me as strange because my understanding of this idea is that it would be undistinguishable from the modern Standard Cosmology.

I'm looking for the reference for the detailed description of this theory, but it should go as follows.

### My sketch of the Cold Big Bang

As we know, the Universe is expanding, so it was smaller in the past. But instead of being hotter, it might have been a very cold and dense neutron star-like state which later expanded and heated up by nuclear decay of neutrons. In the end, the Universe will reach equilibrium state as we know it and will seamlessly merge with the history of Standard Cosmology, leaving small amounts of baryons comparing to light electrons, neutrinos and massless photons.

I see the following attracting features of this idea:

1. We really have no proof that the temperature of the Universe ever was above $\sim 10 MeV$ (number comes from primordial nucleosynthesis as we know no other way to produce elements from protons and neutrons)

2. This picture has a natural matter-antimatter asymmetry: neutrons are everywhere and there are no anti-neutrons.

3. Binding energy of the neutron is small, so decays will slowly build up the temperature while non-relativistic nature of the system will control the expansion speed as $\frac{\ddot{a}}{a} < 0$

### Questions

1. What is the original description of this theory by Lemaître and contemporaries?

2. Will this cold neutron state be stable against collapse? How dense it should be?

3. Is it possible to reach $~MeV$ temperatures and equilibrium?

4. What would be the observable differences of this picture — i.e., why it was disproven?

• The 1931 version of Lemaitre's paper can be found here. It is basically the usual FLRW metric though IIRC he considered the special case of a closed universe. His paper does not describe a universe expanding outwards from a point as you suggest, and indeed the Big Bang did not happen at a point. – John Rennie Jun 11 '16 at 9:20
• – John Rennie Jun 11 '16 at 9:21
• @JohnRennie thank you for the link to the paper — now I see I could not find it because of the wrong keywords! P.S. I never suggested this, I only mention Universe being smaller in the past – Andrii Magalich Jun 11 '16 at 9:43
• Lemaitre did not describe a cold start to the universe. The temperature in the Lemaitre theory varies in the same way as the FLRW metric i.e. it goes to $\infty$ as $t\rightarrow 0$. – John Rennie Jun 11 '16 at 10:38
• @JohnRennie I still need to read the paper, but here is one of my references en.m.wikipedia.org/wiki/Cold_Big_Bang . Wikipedia might be wrong, but I believe that I've read about this in a book about the history of GR or astronomy - unfortunately, I can't provide a reference now – Andrii Magalich Jun 11 '16 at 10:57