Question about the size of atoms as time progressed

[Kuni]

TL;DR Summary

The size of atoms should have expanded at the same as the expansion of space time

My understanding from reading about the big rip is that matter itself will be torn, aka ripped, apart in the very last moments of the universe's existence as the expansion of the fabric of space-time mega-accelerates in those last fractions of a second.

While the big rip is still a hypothesis, the expansion of space time is not.

Which begs the question: How "tight" (or smaller) were atoms, say around 12 billion years ago, when the universe was much smaller?

The universe, aka space-time, has expanded significantly since what we call the big bang, meaning that mater itself should have also expanded at the same rate. The orbits of electrons should have been a lot closer to the nucleus than they are now, and the size of those electrons should also have been a lot smaller. The same applies to protons and neutrons in the nucleus, they should also have been packed tighter together, along with their also being a lot smaller.

Thanx in advance.
Kuni Leml

P.S. Yes, I tried Googling it, but found nothing relevant.

 

[Ibix]

Atoms and other bound systems, up to and including galactic clusters, do not expand with the expansion of the universe. This is basically because the expansion is basically inertial motion, complicated by the curvature of spacetime, and bound systems have some interaction strong enough to have stopped their initial separation. So atoms were always the same size.

The Big Rip is a hypothesis in which quintessence, a variant of dark energy, causes run away expansion in the far future. It has no effect in the current epoch or at any point in the past. It's only in a short period before the Big Rip that it has any effect on the atomic scale - the Wikipedia article quotes an example universe where the Big Rip happens hundreds of millions of years in the future. Atoms are torn apart only 10^-19s before the end.

So, in short, atoms have always been the same size since they first started forming after the universe cooled enough to allow them, and always will be. Even in a Big Rip scenario, this is true up until a tiny fraction of a second before the end.

 

[PeroK]

The universe, aka space-time, has expanded significantly since what we call the big bang, meaning that mater itself should have also expanded at the same rate. The orbits of electrons should have been a lot closer to the nucleus than they are now, and the size of those electrons should also have been a lot smaller. The same applies to protons and neutrons in the nucleus, they should also have been packed tighter together, along with their also being a lot smaller.

Another way to think about this is that a hydrogen atom forms by a proton and electron binding via the electromagnetic force. That is a local process and doesn't depend on how the universe as a whole has evolved. If you put a proton and electron together today or 10 billion years ago or 10 billion years into the future, you will get the same hydrogen atom.

The expansion of space is a large-scale process determined by the overall density of the universe. The equations that govern universal expansion cannot apply to something like an atomic system. An atomic system does not meet the same criteria (in terms of mass density) as the universe does on a large scale.

 

[Vanadium 50]

How "tight" (or smaller) were atoms, say around 12 billion years ago, when the universe was much smaller?

I think you need to explain how you measure this. You can't take a ruler back in time and you can't take an atom from the past to the present. If you can't think of a way, even in principle, to make the comparison, it's not really a scientific question.

 

[Kuni]

So, in short, atoms have always been the same size since they first started forming after the universe cooled enough to allow them, and always will be. Even in a Big Rip scenario, this is true up until a tiny fraction of a second before the end.

Thank you for your response.

First, I will say "that would have been my answer a year ago" (but in extreme layman's terms.) And seeing at that no else has commented differently during the past two months, I will assume that it is still the consensus.

When I first read your response right after you posted it, I was going to just thank you and move on. But something was bothering me, so I decided to find out how this idea got into my head. I did go to trouble of creating an account here and asking a question that I knew was contrary to the mainstream and that could be described by the ungenerous as 'dumb.'

So I started wondering where I got this idea from and decided to find out first before responding. After watching reruns of both BBC-E's Horizon and Discovery Science's "How the Universe Works" over the past couple months whenever they were on, I found it.

Discovery Science's "How the Universe Works" Season 7 Episode 7 "Battle of the Dark Universe" around 10 to 15 minutes near the end, one of the physicists/astrophysicists mentions that 'protons would be ripped apart' or something to that effect.

Now I am cognizant of the fact that the Big Rip is merely a hypothesis, but it still does not change my question, re-watching that episode actually has me looking for more clarity.

• Is the consensus that atoms are not expanding with the fabric of space, because "the yardstick" used to measure the size of atoms is also expanding with the fabric of space?

Because then atoms would have been smaller a few billion years ago. And that smaller size, relative to the current size, could be calculated as a percentage, based on the rate of expansion of space since then.

• Or, is the fabric of space expanding, but the matter, that occupies space/rests on the fabric of space, is not expanding along with space?

I am aware that your answer would appear to imply a "yes" to the latter question, but I just want to confirm that, so that I can finally get this foolishness out of head that has been bugging me since probably the first time that "How the Universe Works" episode originally aired early last year.

Thank you in advance.

 

[Vanadium 50]

Coming back and repeating your question without addressing the points raised is not conversation.

I think you need to explain how you measure this. You can't take a ruler back in time and you can't take an atom from the past to the present. If you can't think of a way, even in principle, to make the comparison, it's not really a scientific question.

 

[Nugatory]

So I started wondering where I got this idea from and decided to find out first before responding. After watching reruns of both BBC-E's Horizon and Discovery Science's "How the Universe Works" over the past couple months whenever they were on, I found it.

These are written to entertain people who are interested but not interested enough to invest the effort needed to learn the real thing. There’s nothing with that as long as you know what you’re getting, but you have to understand that you are not getting the real thing.

Or, is the fabric of space expanding, but the matter, that occupies space/rests on the fabric of space, is not expanding along with space?

That is an OK heuristic picture of what’s going on, but be warned that this notion of a “fabric” will eventually lead you astray.

 

[PeroK]

@Kuni it's important to realize that the big rip hypothesis is very different from the standard model of cosmological expansion. No popular science program can resist presenting the big rip and that creates a false impression that the big rip is already taking place.

One of the major problems with popular science programs is that they blur the boundary between established theories and extravagant speculation.

 

[PeroK]

PS Horizon is especially bad at this. I can practically write their scripts:

Meanwhile, in a remore laboratory beneath the Sahara Desert, there's a physicist who thinks that perhaps everything we believe about the origin of the universe is wrong. If he's correct, it will be the biggest shock to the scientific community since Einstein discovered that the speed of light is finite.

 

[Drakkith]

• Is the consensus that atoms are not expanding with the fabric of space, because "the yardstick" used to measure the size of atoms is also expanding with the fabric of space?

I believe such a size change would be observable as a change in the energy levels in various atoms. I don't know how much of a change this would make, but it might be observable. @Nugatory @PeroK Am I anywhere in the ballpark of correctness here?

Because then atoms would have been smaller a few billion years ago. And that smaller size, relative to the current size, could be calculated as a percentage, based on the rate of expansion of space since then.

The rate of expansion has been slowing down over time, so they should actually be larger in the past, not smaller.

 

[PeroK]

I believe such a size change would be observable as a change in the energy levels in various atoms. I don't know how much of a change this would make, but it might be observable. @Nugatory @PeroK Am I anywhere in the ballpark of correctness here?
The rate of expansion has been slowing down over time, so they should actually be larger in the past, not smaller.

The expansion of the universe takes place on a cosmological scale. It relies on the overall uniform mass density of the universe. That does not apply to a galaxy or solar system, let alone to an atom.

As the universe expands, the vacuum density increases and this increases the overall rate of expansion. But, as the solar system or atom is not expanding in the first place there is no accelerated expansion.

The big rip scenario comes from an alternative theory where space itself changes over time. It requires the concept of quintessence. If this increases exponentially over time then eventually things on the size of an atom will be subject to a quintessential expansion. Whereas, atoms will never be subject to a dark energy expansion, if we assume dark energy is constant per unit vacuum.

 

[Drakkith]

Oh, so dark-energy-driven expansion has zero effect on locally bound systems, but this 'quintessence' is different then? I'm not familiar with the details of the big rip at all.

 

[PeroK]

Oh, so dark-energy-driven expansion has zero effect on locally bound systems, but this 'quintessence' is different then? I'm not familiar with the details of the big rip at all.

Yes, dark energy in the form of a cosmological constant is constant over time. The dynamics of the solar system or an atom will remain the same. But, as the universe expands and the mass density (of the entire universe) reduces, so the effect of the constant increases. You can put the data into the Friedmann equation and eventually you get gravitationally bound islands of galaxy clusters all receding from each other exponentially. But, there is no big rip within these bound systems.

Quintessence is a form of dark energy where the repulsion increases over time to the point where it rips everything apart.

https://en.wikipedia.org/wiki/Quintessence_(physics)

 

[Vanadium 50]

I believe such a size change would be observable as a change in the energy levels in various atoms. I don't know how much of a change this would make, but it might be observable. Am I anywhere in the ballpark of correctness here?

I wish you hadn't gone down that path. It's at best speculative, turns out not to be correct, but worst of all it takes the heat off the OP to answer the question "how do you measure this" by making "heck, I dunno, something with energies" an acceptable answer.

It';s not correct - there's one relevant unit of length in atomic energy levels - the reciprocal of the Rydberg constant. (One could also use the Bohr radius) If I change this, all the energy levels move together, as they are all the Rydberg constant times some number. But we measure wavelengths, and more to the point, measure red-shifted wavelengths. Any change in the Rydberg can be absorbed into the redshift.

Now, though, the odds of getting an answer to "how do you measure this" have gone way down.

 

[Drakkith]

. But we measure wavelengths, and more to the point, measure red-shifted wavelengths. Any change in the Rydberg can be absorbed into the redshift.

Ah, I wasn't aware of this. Thanks, V50.