Nature of the early Universe

[Greta]

TL;DR Summary

Relative density/consistency of the early universe just before atoms could form.

Hello. Wannabe sci-fi writer here with what may be a simpleton's question.

From Google et al: "It took 380,000 years for electrons to be trapped in orbits around nuclei, forming the first atoms. These were mainly helium and hydrogen, which are still by far the most abundant elements in the universe. 1.6 million years later, gravity began to form stars and galaxies from clouds of gas."

Does that mean that the entire humongous expanding universe was, at least towards the end of its first 380,000 years, the temperature and density of a neutron star?

Thank you.

 

[Vanadium 50]

No. What you describe is a plasma, not a neutron star.

 

[Bandersnatch]

The conditions at that point in the early universe are best described as an extremely rarefied gas (something like $10^{-15} g/cm^3$), spread almost uniformly everywhere, and with 3000 Kelvin temperature (about half of the Sun's surface) giving it a yellowish/orange glow.

 

[Greta]

TY for the replies. I still do not understand.

Today, we observe atoms at temperatures far higher than 3000K that are at far greater densities than 10⁻¹⁵g/cm³.

Would someone be able to explain what other factors were in play to prevent atoms from forming during the early universe before 380,000 years? Thank you!

 

[PeterDonis]

Today, we observe atoms at temperatures far higher than 3000K

We do? Where?

that are at far greater densities than 10−15g/cm3.

Yes, because matter in our universe has been gravitationally clumping for billions of years.

Would someone be able to explain what other factors were in play to prevent atoms from forming during the early universe before 380,000 years?

Because if the average temperature is high enough that a random photon hitting an atom will ionize it, atoms will not be able to form and remain in existence. You will have a plasma instead--a mixture of electrons and ions which can't form atoms because the temperature is too high.

 

[Greta]

The temperature in the Earth's lower mantle is about 3000K, and the temps rise towards the core, which I understand is at about 6000C (approx 6270K).

Just plodding my way through this conceptually ... while atoms can exist at 6000C under pressure, they cannot form at much cooler temps than that in an extremely sparse environment. So this is about the distance between particles and their speed?

Based on an earlier reply that space just before year 380,000 would have a density of 10^−15g/cm³. My wonky maths says that's in the ball park of a billion times more dense than interstellar space today. I was trying to work out how dense 10^−15g/cm³ was compared with the Sun's corona, which is 10 million C and also very thin at 10¹⁵ particles/m³. I tried. And failed.

So, summary, the universe was just a thin, energetic plasma, thinner than, say, a fire on Earth. Yes?

I assume that sometime very early on (First second? Early years?), the entire universe would have actually been the density of a neutron star ...?

Thanks again for any further information.

 

[newjerseyrunner]

There seems to be a disconnect between definitions of "atom." What PeterDonis I assume is talking about are neutral atoms, which is what you Greda inadvertently asked about without realizing it.

You (Greda) seem to be under the impression that atoms in their most basic form could not exist before 380K years. That's not true at all. A hydrogen nucleus is just one proton, so these objects existed since about ten seconds after the big bang. The issue is that these protons, could not hold onto an electron. A proton would grab an electron, the instantly be hit by some energy, which would send it flying off again. This means that NEUTRAL atoms could not exist, and that the entire universe was filled with a state of matter called plasma. It's still atoms, they're just ionized where the electrons were free floating.

This is what made the universe opaque. Electrons really like grabbing photons, so any photon coming from anywhere was likely to run into an electron very quickly, hence, obscuring light. At 380K years, the universe had cooled to the point that electrons could finally settle into their energy levels without instantly being popped off. This quickly removed all stray electrons and made the universe suddenly transparent. That sudden flash is the CMB.

 

[PeterDonis]

The temperature in the Earth's lower mantle is about 3000K, and the temps rise towards the core, which I understand is at about 6000C (approx 6270K).

The Earth's core is metallic (mainly iron), whereas the universe at the time of recombination (380,000 years or so after the big bang) was mostly hydrogen (with some helium). Different chemical elements have different temperatures at which they transition between phases (solid, liquid, gas, plasma). The temperature of 3000K or so for transition to plasma is for hydrogen. So it isn't just a matter of "atoms"; you have to look at which specific element(s) are present.

while atoms can exist at 6000C under pressure, they cannot form at much cooler temps than that in an extremely sparse environment

3000K is not "much cooler" than 6000K. It's only a factor of two. Something like room temperature, about 300K, would be "much cooler" than both; but compared to room temperature, both are about the same.

So, summary, the universe was just a thin, energetic plasma, thinner than, say, a fire on Earth. Yes?

Yes.

I assume that sometime very early on (First second? Early years?), the entire universe would have actually been the density of a neutron star ...?

At some very early point (a small fraction of a second after the big bang, I believe, but I haven't been able to find estimates of densities, only temperatures), yes. But remember that the universe was also expanding extremely rapidly then, so it was not the same as a static neutron star.

 

[Greta]

At some very early point (a small fraction of a second after the big bang, I believe, but I haven't been able to find estimates of densities, only temperatures), yes. But remember that the universe was also expanding extremely rapidly then, so it was not the same as a static neutron star.

It's said that the exponential nature of inflation meant that more happened in the first second of the universe than in all the time that has since passed.

In context, I am trying to imagine Planck time freeze frames within that first second. An object with the density of neutronium and the mass of the entire universe. I wonder how big it would be? (for that Planck time)

 

[PeterDonis]

It's said that the exponential nature of inflation meant that more happened in the first second of the universe than in all the time that has since passed.

Can you give a specific reference?

 

[Greta]

Peter, that's a tangential issue. I expect that people around the physics scene would be aware of Lawrence Krauss and his views, paraphrased as:

"One can consider measuring time by the number of events that occur within some period. In this sense, more happened in the first second in the history of the universe than has occurred in the history of the universe since that moment. "

The comment is not meant as a challenge to the status quo, just a consideration the exponential nature of the universe's (and many things') development. I'm just interested in the exotic states of the infant universe and the idea of a single object so dense, large and unstable.

 

[PeterDonis]

that's a tangential issue

No, it isn't. See below.

I expect that people around the physics scene would be aware of Lawrence Krauss and his views

His views expressed in actual peer-reviewed papers, or his views expressed in pop science books, articles, videos, etc?

If it's the latter, it's not a valid reference for PF discussion. That's why you need to give a specific reference.

just a consideration the exponential nature of the universe's (and many things') development

And if these claims about such "exponential nature" having implications about "the number of events that occurred in some period" (what are "events" anyway? how do we count them? how do we know how many "events" happen in a given time?) only appear in pop science sources, they're not valid scientific claims, at least not as far as PF discussion is concerned.

I'm just interested in the exotic states of the infant universe and the idea of a single object so dense, large and unstable.

And if you're interested in whatever actual scientific hypotheses exist, that's fine. But you have to be careful not to confuse claims made only in pop science sources with actual scientific hypotheses.

 

[Ibix]

Can you give a specific reference?

Peter, that's a tangential issue.

This is a general problem with science communication. The maths used by professionals is precise, but there are often competing mathematical models. Describing any model in ordinary language loses a lot of precision, and there are endless variations on how to describe any one. The best way is basically a matter of personal preference. So working backwards from someone's paraphrase of a scientist's description of the bits of a model he thinks are important to figure out which model he's actually talking about is near impossible. That means that we can't really help because we've no more idea about which model we are discussing than you do. (Peter's last post includes a lot of the "no idea what he means by this" problems that follow from your description.)

If you can find your source, you may well find that Krauss says something like "in 2012, Smith and Jones published a model of the early universe that says <blah>". Then someone may be able to go and find Smith and Jones' publications in 2012 and find out what Krauss is actually talking about. Then we can discuss it usefully.

 

[Vanadium 50]

I assume that sometime very early on (First second? Early years?), the entire universe would have actually been the density of a neutron star ...?

At some very early point (a small fraction of a second after the big bang, I believe, but I haven't been able to find estimates of densities, only temperatures), yes.

I get about t = 6 hours.

However, even though the density was the same as a neutron star, the environment was nothing like a neutron star. It was radiation dominated, and had a much lower baryon density than a neutron star does today. Indeed, nuclei had already formed by that time and were sufficiently far away from each other not to interact.

 

[Greta]

Thanks Vanadium. Fascinating info. I don't suppose the math extends to the universe's possible size at after six hours?

 

[phinds]

Thanks Vanadium. Fascinating info. I don't suppose the math extends to the universe's possible size at after six hours?

We don't know the size of the universe, now or then, but as has already been pointed out in this thread the general belief is that it is most likely infinite which of course means that it has always been infinite.

EDIT: I had another thread in mind when I said that had already been pointed out here.

 

[Greta]

The universe? It depends on whether you are referring to 1) the hypothesised quantum foam that preceded the BB or 2) the creation and expansion of spacetime.

While experts see the "singularity" as being purely theoretical, it is still well-established that the universe(2) was much smaller than it once was, maybe a quanta, a Planck volume or smaller. But it was not infinite at that time, even if the potentials of spacetime are seemingly infinite.

So I'm still interested in the possible size of the universe(2) after six hours.

 

[phinds]

But it was not infinite at that time, even if the potentials of spacetime are seemingly infinite.

Citation, please. You have stated something as a fact that is NOT a known fact. It may be true or it may be false, but stating it as true is incorrect.

 

[PeterDonis]

it is still well-established that the universe(2) was much smaller than it once was

No, it is well established that our observable universe was once much smaller than it is now. But our observable universe is not the same as the entire universe.

 

[Greta]

The question in post #17 was not asking about broader existence that triggered the big bang. That would be utterly pointless. I am obviously asking about the phenomena that commenced with the BB, not that whih triggered it.

The continuing big bang that is commonly called "the universe" is obviously much larger than the observable universe, so the latter has nothing to do with my question.

I have heard scientists speak about when the universe was the size of a soccer ball. My question goes the other way, wondering about its scale (using the common definition of "the universe" as the phenomenon that commenced with the big bang) after six months. Given speed of expansion, it would have already been humongous - but not infinite.

 

[phinds]

https://www.physicsforums.com/find-threads/unanswered

The continuing big bang that is commonly called "the universe" is obviously much larger than the observable universe, so the latter has nothing to do with my question.

I have heard scientists speak about when the universe was the size of a soccer ball. My question goes the other way, wondering about its scale (using the common definition of "the universe" as the phenomenon that commenced with the big bang) after six months. Given speed of expansion, it would have already been humongous - but not infinite.

Again, this is not true. You really need to decide what defintion you are using. Here you specifically say you are talking about the universe, meaning the whole universe, which might be infinite in extent, but you are trying to describe it using a statement (not infinite) that potentially only applies to the observable universe.

 

[PeterDonis]

The continuing big bang that is commonly called "the universe" is obviously much larger than the observable universe, so the latter has nothing to do with my question.

If you are not asking about the observable universe, then this...

I have heard scientists speak about when the universe was the size of a soccer ball.

...is irrelevant, since the "universe" these scientists are talking about when they say this is the observable universe, not the entire universe. According to our best current model, the Lambda CDM model, the entire universe is spatially infinite. That's the best we can do right now since our observations are not accurate enough to distinguish between a spatially infinite universe and a spatially finite universe which is so large that its spatial curvature is very small.

 

[timmdeeg]

... That's the best we can do right now since our observations are not accurate enough to distinguish between a spatially infinite universe and a spatially finite universe which is so large that its spatial curvature is very small.

Important note!

 

[timmdeeg]

I have heard scientists speak about when the universe was the size of a soccer ball.

https://fr.wikipedia.org/wiki/Big_BangThe soccer ball means the size of that patch of the universe at that time (*) - when inflation ended and matter condensed out - which then expanded to the size of our observable universe. At the big bang said patch has been much much smaller and expanded driven by the inflation by a factor of 10²⁶ to the size of a soccer ball .

(*) The soccer ball size should be understood cum grano salis. As far as I can tell this size depends mainly on the duration of the inflation.

 

[Greta]

Thanks Tim, for acknowledging the question.