Is the Universe the Size of a Grapefruit?

[antd]

I have read in many places that the universe was once smaller than an atom. However, many people here have said this is a 'cartoon' version of the big bang theory. And that the media have popularized this invalid notion...

I now have some questions, can I have the 'true' answer (what we know now) and not the cartoon answer please :)

1. Was the universe the size of a grapefruit at some point in the distant past? (some say the observable universe was, but if the observable universe is isotropic, then surely the whole universe is basically the same? And therefore the whole universe must have been small in the past)

2. What is the shape of the universe?
(I've read that it is probably flat)

3. Is the universe infinite? (If the universe is 'space' there surely can't be an infinite amount of it? but I guess the universe is space-time, and time in theory can go on forever?)

4. Why isn't the space between me and my bed expanding? Why isn't the space between my atoms in my body expanding?

5. Is my body made of dark matter? What is made of dark matter? (I've read that dark matter makes up empty space, but my body has empty space etc...)

Sorry if my questions are dumb. I would just like to get a better understanding of what the general concenus among scientists is.

Thanks!

 

[Ich]

1. Was the universe the size of a grapefruit at some point in the distant past?

Maybe.
The observable universe once had the size of a grapefruit. The total universe may be much bigger or even be infinitely big. iIf it's infinite now, it was always infinite. We simply don't know for sure, as it is not observable.

2. What is the shape of the universe?
(I've read that it is probably flat)

The shape of space, as defined in cosmology, is flat. That does not mean 2D, it means 3D euclidean.

3. Is the universe infinite? (If the universe is 'space' there surely can't be an infinite amount of it?

Your questions always meant "space", not spacetime. Space can be infinite. Spacetime most likely is, as we expect the universe to be infinite in time.

4. Why isn't the space between me and my bed expanding?

Expansion is not a physical effect, it's rather an initial condition: If the distance between you and your bed was expanding in the past, it will continue to do so. If not, not. Some, but not nearly all, call such behaviour "velocity".
If there is either repulsive or attractive material between you and the bed, or if you and the bed gravitate, this motion will change with time.
You can extrapolate the scenario to the whole universe, where it may be advisable to drop the "velocity"-notion and talk about expanding space instead. At least in a closed universe.

5. Is my body made of dark matter?

Not if you mean cosmological DM. DM is thught to be kind of a gas, which can pervade eveything. Neither does it make up space nor is evenly distributed in space.

 

[mikeph]

I'd slightly differ with some of these questions to Ich, in that the space between you and your bed is expanding due to cosmological factors, (in a general uniform sense, ignoring the idea that we may experience less expansion being in a galaxy cluster with a local density greater than average), but your bedroom has internal binding energy. This is a key point and the reason that the universe can expand by a factor of a thousand, but the atoms contained in the universe are no larger than when they first formed from protons and electrons. The forces at work governing the minimum energy radial position for an electron to orbit a proton are no different, and if you have a little bit of expansion, then the electron may be shifted out slightly, but it'll just fall back in. The main objects that are affected by the expansion are photons, who, having no internal binding energy, are stretched mercilessly by the expansion and are now about 1000 times longer wavelength that when they began traveling at decoupling early in the universe's life.Also, Your body is probably not made of dark matter, though neutrinos are inside your body now and are considered hot dark matter (due to their speed), a lot of dark matter is in space. You are not dark matter because you are visible- dark matter by definition couples to gravity but weakly or not at all to electromagnetism, and a lot of it is probably just cold dust. There are some unknown ideas, that it could be super symmetric particles or heavy neutrinos, but there isn't much evidence for this. Dark energy is the one you should watch out for!

 

[Ich]

the space between you and your bed is expanding due to cosmological factors

There are the gravitational effects I described (DE and matter between you and the bed) which provide relative acceleration. Besides that, expansion is a matter of initial conditions.
If there is no DE, you need no force to keep the distance between you and your bed from expanding - if both start at relative rest. (I won't discuss this issue here further, as it makes a lot of people very angry and is widely regarded as a bad move.)

 

[S.Vasojevic]

The forces at work governing the minimum energy radial position for an electron to orbit a proton are no different, and if you have a little bit of expansion, then the electron may be shifted out slightly, but it'll just fall back in.

Then expansion should raise temperature of matter, or not?
Also we can build device to extrapolate energy from expansion. It should probably be larger then solar sistem to give us some resonable amounts of power.

 

[mikeph]

I don't know anything about the thermodynamics of cosmological expansion although I would agree that if something is expanding the universe, they are doing a hell of a lot of work separating galaxy clusters, unless I am missing something.

 

[S.Vasojevic]

My guess is that if expansion suddenly stops, electrons would just shrink their orbits just a little bit. I doubt that they are constantly "compensate" for expansion, if they do they would be performing work.

 

[mikeph]

They are not constantly compensating, the idea is that as space expands, the electron is displaced from equilibrium momentarily. The expansion is doing to "work", not the electron.

 

[antd]

But how can it be infinitely big? If the universe is infinite how can it make sense to say the universe is expanding?

If something is infinite surely it doesn't need to expand, since it cannot get any bigger? And if it is infinite, surely we cannot know if it is expanding?!

I read from NASA that the universe probably is flat within 2% margin of error (according to results from WMAP)

What does it mean for the universe to be infinite? Does it mean that one could always keep adding particles?

Even if we theoretically 'freeze' the expansion? Will it still be infinite?
Also, does it mean there are also an infinite number of atoms in the universe? Infinite number of human beings?

I don't understand 'infinity' whatsoever ;) It seems infinity only exists in mathematics and not in reality.

Too many questions, sorry :P

 

[mikeph]

But how can it be infinitely big? If the universe is infinite how can it make sense to say the universe is expanding?

If something is infinite surely it doesn't need to expand, since it cannot get any bigger?

Think about the set {...-2,-1,0,1,2,3...}, "infinite" in size. What about the set {-4,-2,0,2,4,6...} ? Twice as large spacing between elements. How about {...-2*a(t), -a(t), 0, a(t), 2a(t), 3a(t),...} ?

Do you agree that if we increase a over time, the set will always be infinite in size, but more distantly spaced? In cosmology a(t) is a useful factor to describe the relative sizes of the universe at different times.

 

[antd]

Yes, but this is a kind of concept which exists in the mind. It doesn't describe something that exists in reality.

 

[Freeman Dyson]

At this point, infinity does only exist in mathematics and not in "reality". When an infinity pops up in a theory the scientist takes it to mean the theory is incomplete. Like GR with quantum effects. A proper theory of quantum gravity would get rid of those infinities. Smooth them down to the finite. And if the universe were infinite, it would take an infinite amount of time to figure out that it was infinite. Could we ever really be sure if the universe was infinite or just really, really big?

 

[S.Vasojevic]

They are not constantly compensating, the idea is that as space expands, the electron is displaced from equilibrium momentarily. The expansion is doing to "work", not the electron.

I don't think so. If they are instantaneously displaced back and forth then there is infinite work, if the space expands with no displacement, where is then the limit when expansion catches particles. I think that equilibrium is with expansion already accounted for, or that possible electron orbits are not exactly the same with or without expansion.


And yes, infinite number of atoms, or infinite mass is a problem. Not for expansion, as MikeyW pointed, but for many other things.

 

[S.Vasojevic]

And that of course means, that if expansion would accelerate many orders of magnitude, that it could be capable to rip appart every atom in universe.

 

[antd]

Then it seems that we don't know much about the Universe :P

I don't know why anyone would say the universe is infinite though. Since it seems infinite cannot exist in reality. Shouldn't all scientists say 'our current theories are not compatible' or something to that extent...

And yes, I think if our technology improves to be incredibly good in the future, then we perhaps could somehow infer its approximate size.

 

[Freeman Dyson]

From what I know, scientists don't like infinities and when they pop up they are seen as an indicator of a flaw in the theory.

 

[S.Vasojevic]

Cosmological principle states that universe is, on largest scales, homogeneous and isotropic, and same for any observer.
There are two ways to achieve that. If universe is flat on largest scale, then it must be infinite to look same for every observer.
Other way is that it is slightly curved, and which every way you go it will look the same, but it wouldn't be infinite in content.
From observations carried so far it appears to be flat, but there is always possibility that it is so slightly curved that we can not detect that yet.

 

[antd]

But this 'infinite', it seems it cannot really exist...
Are you saying it is infinite because the expansion is faster than the speed of light? (in the flat universe)

Surely it's a flaw in the current physical model, which is leading to this infinite

 

[S.Vasojevic]

Well I think that is why people hope that eventualy it will turn out that it is curved. If not, it would mean, among the other things that there are infinite number of me and you discusing is it, or is it not infinite.
There is also a third possibility which is HIGHLY unlikely, and that is, that universe is flat, finite, but somehow we have special position in it, so it appears same in every direction, just for us.

 

[Ich]

But this 'infinite', it seems it cannot really exist...

I think almost everytime philosophers stated that something cannot exist, nature made clear that it is not philosophy. Don't tell the universe what to be.
That said, we actually know something about the observable universe and, by definition, nothing about the unobservable. If you have a problem with infinity, think of it as finite. It doesn't matter, at least not until we get a handle on the yet unobservable, too.

 

[diogenesNY]

Things tend to get very funky and non-intuitive when dealing with infinities. I think that it is worth noting that an infinite universe does not necessarily imply an infinite number of 'you's and me's ', nor an infinite configuration of things or probabilities. A real mathematician could undoubtedly describe the different 'types' of infinities far better than I can, however my previous proposition can be pretty trivially demonstrated:

We can look at an infinite sequence of numbers (choose your set: integers, rational, real... its up to you) and it is observably clear that this infinite sequence of numbers does not contain _all_ numbers. In fact, most (possible all..?) cases, you can fit further sets of infinite identifiable sequences of numbers between the numbers of your originally chosen infinite sequence... whatever sequence you originally chose. And it gets even funkier than that , but I am already tiptoeing beyond my ability to reckon these matters... perhaps a real mathematician could pick up here...

diogenesNY

P.S. Obligatory math humor:

ALEPHnull bottles of beer on the wall,
ALEPHnull bottles of beer,
you take one down,
pass it around,
ALEPHnull bottles of beer on the wall.

Well I think that is why people hope that eventualy it will turn out that it is curved. If not, it would mean, among the other things that there are infinite number of me and you discusing is it, or is it not infinite.
There is also a third possibility which is HIGHLY unlikely, and that is, that universe is flat, finite, but somehow we have special position in it, so it appears same in every direction, just for us.

 

[mikeph]

I don't think so. If they are instantaneously displaced back and forth then there is infinite work, if the space expands with no displacement, where is then the limit when expansion catches particles. I think that equilibrium is with expansion already accounted for, or that possible electron orbits are not exactly the same with or without expansion.


And yes, infinite number of atoms, or infinite mass is a problem. Not for expansion, as MikeyW pointed, but for many other things.

There is not infinite work- when you talk of instantaneous displacements you need to consider also that the amount of work done in each instant is also tiny. I don't follow your reasoning. And I don't think this is how I was taught... the expansion is an expansion of the space time and is not already accounted for by potentials of, say, the electron.


Infinite space or infinite mass is not a problem, as extensive variables. Infinite density or temperature is a problem, as intensive variables.

 

[S.Vasojevic]

Well I didnt say that electron charge is different, I said that orbit is not exactly the same. Non relativistic electron velocity is given with V = ZαC, so we can maybe discuss light elements with small Z (atomic number), without engaging into relativistic quantum mechanics. I am not claiming I am right, but I just can not see how something with charge and mass can move instateniosly. So if we take hydrogen we get V = αc, which means that my claim is that alpha is not the same with or without expansion. I am on thin ice, because I suggest that some fundamental constant is changing, but I will stick to it until somebody disapproves me.
Anyhow, at Bohr radius we are dealing with recession velocity of 1.2176 e-28 m/s, so even if I am right the change would be so minuscule, that we could not possibly detect it.

Anyone? Any thoughts? How does electron deals with expanding space?

BTW I am having trouble with this latex thing, why do lowergreeks comeout superscripted?

 

[S.Vasojevic]

Things tend to get very funky and non-intuitive when dealing with infinities.

I can only speculate, but it would depend from perspective. From our point there is only one universe, and it is finite. What lays beyond is open, and depends only how far and fast you are able to go. But we can suspect that beyond our horizon lays the truly infinite universe.
As Ich nicely put it, all we can say is that observable universe was once size of a grapefruit.

 

[Chronos]

Irrelevant, expansion has no effect at atomic levels, or even galactice cluster levels for now. That may change in the very distant future, but, not any time soon. Our best observational evidence to date only hints at a possible 'Big Rip' scenario. I agree with Vas and Ich, the obervable universe is the only universe accessible to us. It may be infinite, but, that is unprovable at present. The observable universe is presently 13.7 light years in every direction [in light travel time].

 

[Ich]

Anyone? Any thoughts? How does electron deals with expanding space?

Because nobody else answers:
It doesn't deal with expanding space. As I said, expansion is a result of initial conditions. A hydrogen atom is in one definite state, independent of the exact initial conditions. It doesn't matter whether the universe is expanding or not.
If there is Dark Energy (as it seems), and DE is thought to be a smooth background at atomic scales, you can add it as a static negative parabolic potential to the Coulomb potential. This means that the electron's "orbit" is a little bit different and the electron is a little bit less tightly bound. If DE were much, much stronger, the effective potential could be such that the electron is no longer bound at all. That's what chronos refers to as "Big Rip".

 

[S.Vasojevic]

Well I usually get in trouble saying just expansion, instead accelereted expansion. But that is it. So my guess was right. Thanks.

 

[BigFairy]

That sounds about right to me.

 

[Skolon]

The observable universe is presently 13.7 light years in every direction [in light travel time].

Chronos, I'm sorry but you are wrong. The edge of observable universe is today at 46.5 billion ly away (so, the diameter of observable Universe is now 93 billion ly). That is the effect of expansion.
The Universe has began 13.7 billion years ago, but its radius is larger than 13.7 billion ly.

But if you say that the first light of the Universe traveled 13.7 billion years, then you are wrong again. First seen light appeared after 350.000 years after BB, so the "first" foton have just 13.3-13.4 billion years.

 

[mikeph]

Hold on, 1 billion = 1,000,000,000, so 13.7 bn years minus 350,000 years is not 13.3 bn years! It's 13.69965, which is close enough for me!

 

[Skolon]

Hold on, 1 billion = 1,000,000,000, so 13.7 bn years minus 350,000 years is not 13.3 bn years! It's 13.69965, which is close enough for me!

Yes, you are right, I was wrong. The 13.7 billion is the correct value.

 

[Chronos]

Obviously the universe is 'now' much larger than 13.7 bly, how much larger is unknowable, imo. It would require knowledge of the speed of exansion NOW across all spacetime intervals. I fail to see how this is possible to achieve. Size based on light travel time, however, is an empirical measurement.

 

[S.Vasojevic]

One thing is not clear to me though, is Hubble law calculating distance as we see it (light travel time), or "now" time. Because when you plug C for recession velocity you get distance of 13.7 Gly, so it appears to me that it calculates distances based on light travel time.

 

[justwondering]

I have read in many places that the universe was once smaller than an atom. However, many people here have said this is a 'cartoon' version of the big bang theory. And that the media have popularized this invalid notion...

I now have some questions, can I have the 'true' answer (what we know now) and not the cartoon answer please :)

1. Was the universe the size of a grapefruit at some point in the distant past? (some say the observable universe was, but if the observable universe is isotropic, then surely the whole universe is basically the same? And therefore the whole universe must have been small in the past)

2. What is the shape of the universe?
(I've read that it is probably flat)

3. Is the universe infinite? (If the universe is 'space' there surely can't be an infinite amount of it? but I guess the universe is space-time, and time in theory can go on forever?)

4. Why isn't the space between me and my bed expanding? Why isn't the space between my atoms in my body expanding?

5. Is my body made of dark matter? What is made of dark matter? (I've read that dark matter makes up empty space, but my body has empty space etc...)

Sorry if my questions are dumb. I would just like to get a better understanding of what the general concenus among scientists is.

Thanks!

"I have read in many places that the universe was once smaller than an atom."

Said to be even smaller than a Proton at one instant! It just gets sillier and sillier!

 

[friend]

"I have read in many places that the universe was once smaller than an atom."

Said to be even smaller than a Proton at one instant! It just gets sillier and sillier!

If the universe did not come from a singular point, then that brings up questions of causality; how could everything come from absolutely nothing? But is easier to think that the universe came from nothing when if it began from a singularity, because a singularity can be considered nothing, since there is no dimension to it.