Size of the Universe

I've been having a difficult time trying to find a clear answer, or at least somewhat clear, as to what the actual size of the universe is. I presume that the observable universe is about 30 billion light years in diameter, since we can see about 15 billion light years in each direction. Please correct me if I'm wrong on this.

I've read that the universe was about 8 light years in diameter and ten billion degrees Kelvin one second after the Big Bang. I've also read that when the universe doubles in size, its temperature halves. Because we know that the temperature of the universe is currently 2.7 degrees Kelvin, would it not be possible to determine the diameter of the entire universe (assuming that it has a flat geometry on average)? If I'm approaching this problem the wrong way, please correct me.

 

Log in or Sign up to hide all adverts.

Hm , a interesting and seemingly obviously problem.

<font size=2>W</font>e can find out the age and size of the Universe in a number of ways.
(Starting with the basics)
By finding the age of the oldest stars we see. Our knowledge of the evolution of stars is very good . (<i> and using parallax</i> ).
A second way to get an age for the Universe is to try to figure out the time of the BB.
By finding out how bright things (<i>like Cepheid variables</i>) appear to be - to determine the distance of the nearby galaxies , and relating that value to their red-shift .
<i>The relative velocities caused by the expansion of the Universe begun with the BB. </i>
So, once we've found out how expansion velocity correlates with distance , we can extrapolate back (with some assumptions) to calculate the start time of the big bang, (when all the matter in the Universe was at one point) and using the present temperature extrapolate the size of the universe.

The latest results from the <b>WMAP</b> probe show that the Hubble Constant to be 71 km/sec/Mpc, +0.04/-0.03. and age to be 13.7 billion years old.

And it would be a <b>simple</b> case like you said to determine the initial temperature and size today...

<b>Unfortunatly</b>,

Theory says that during the BB a very quick '<i>inflation</i>' happened, (in an infinitesimal fraction of a second the space and time became extremely large). At the <b>Planck time</b>, <i>symmetry breaks</i> and gravity becomes a distinct force. This is the fabled <b>GUT</b>s era. ( <i>1x10^-43 seconds</i>)

<i>Space expanded faster than the speed of light</i>.
So large that space also became very flat (to 10⁶⁰ decimal places!).

This is the Quantum limit (<i>10^-35cm - Planck length</i>) of classical general relativity where nothing makes sense! The Temperature equals 10³²

During the Inflation Era the limits of perturbative interaction - (<i>thermalization of universe, Time=1x10^-38 seconds</i>) the Temperature was 10²⁹

The cosmic background radiation was created by matter and anti- matter particle annihilated ,(<i> photons</i>).
The number of photons is the same now as then. And in a ratio of a <i>billion to one</i> to protons; and the energy of the photons steadily drops as the universe expands.

The temperature of this annihilation radiation was <b>greater</b> than a trillion degrees Kelvin, (and the universe has also expanded by a trillion times since that era).

The background radiation therefore is a trillion times less (2.7 K )

Once inflation was over, the universe continued to expand and cool.
When it was at a temperature of 1000 GeV (about 10 thousand billion degrees), quarks were formed, (and elementary particles )

So it seems that the inflationary period has <b>hidden</b> the true size of the universe.

Perhaps there may be another way we can find out the size of the universe. by using how flat the universe is...We know to how many decimal places the flatness is (using QCD)
(If the visible universe is at the critical density, then the total mass of the universe is about 1⁵³ kg, and the number of atoms is about 6⁷⁹.)
We could say that the smallest size that the universe could be was of the order of 10⁶⁰ (magnitudes ) .? ( smallest sphere, with a flat surface, size?)...

 

Log in or Sign up to hide all adverts.

Thank you, blobrana. That is the most helpful statement I've read concerning why I can't find any source that will speculate as to the size of the entire universe.

I still don't quite understand exactly how inflation has hidden the true size of the universe though. Do we not know how much inflation should have expanded the universe? I've read in several places that during inflation, the universe swelled from something like the size of an atom to the size of the solar system. If our estimates are not far off, can we not then use the size and temperature of the universe after inflation and extrapolate the size of the universe today based on its current temperature? Perhaps you've already answered this, but if you did, I didn't follow. Thanks for your input!

 

Log in or Sign up to hide all adverts.

Btw, thanks for mentioning this. I found the WMAP site to be very informative and helpful.

 

It's not even inflation that limits our knowledge of the universes size - standard cosmology does. From light we can tell the age of the universe, but the size is limited to our visible region of the universe. In other words, we can find out the size of the visible universe, but anything outside that sphere is much bigger, and of course hidden. For all we know, the universe could be infinite but we will only ever be able to see a small part of it.

 

Inflation

As far as the amount of inflation, it depends on the scenario. There are many models out there,and each author claims his is the correct one. En vogue these days are scalar field and potential driven models but they are plagued by problems, mainly the cosmological constant problem which they're trying to solve now with results like the WMAP survey. Another interesting scenario I've seen is T. Padmanbhan's "Inflation from quantum gravity" in which the inflation is produced from gravitational interactions. Inflation proceeds from a Planck size "bubble" (~10^-33 cm.) and inflates ~e^e^10^13 times with the result the universe is a "bubble" with a "diameter" of ~= 10^10^{20581041168255} centimeters! (light years, parsecs, vendekameters, take your pick)

 

I understand that the speed of light limits the amount of the universe we can see. But should we not be able to estimate the size of the entire universe based on the information we have? My logic goes as follows:

1) 5.31 x 10^-44 seconds (Planck time) after the Big Bang, the mass of the entire universe was packed into an area 1.62 x 10^-35 metres (Planck length) in diameter. Thus we know with some certainty the size of the entire universe at this time.

2) Inflation happens. The universe expands by a factor of about 10^50 before inflation ends. Because we know approximately what factor the universe expanded by, we can estimate the size of the entire universe at this time.

3) The temperature of the universe halves every time the size of the universe doubles.

4) We have an estimate of the temperature of the universe after inflation ended (about 10^27 to 10^28 Kelvin).

5) We have an estimate of the size of the universe after inflation ended.

6) We know the temperature of the universe today.

7) Therefore we should be able to estimate the size of the universe today. So what is this estimate???

If I have erred in any part of this, please correct me.

 

The entire visible universe.

 

Please Register or Log in to view the hidden image!

That doesn't make sense to me because the amount of mass in the observable universe gets smaller every day since space is expanding and galaxies near the horizon are dropping out of sight. From this it would seem that the total mass of the observable universe decreases as time goes by. Therefore it doesn't make sense to say that it was the mass of the observable universe, a number which has continually decreased over the last 13.7 billion years, that was crammed into that volume with a diameter of Planck length at Planck time.

 

Why should it make a difference? Just take the visible universe today, and calculate what happens when you run the clock back on this region of space 13.7 billion years.

In the case of an infinite universe, the same would apply. Run the clock back on our visible universe, and it becomes very small and dense. But outside this visible region, there is still an infinite volume of space. Now I'm guessing this is the same for a finite universe, but I could be wrong.

 

ON the nail.

The visible universe as far as i know was once a false vacuum that has inflated to greater size of our visible universe.
This false vacuum bubble is of course our universe. And the bubble has a bubble-wall.
We do not see this wall in our visible universe but there is no way of knowing if we (our universe) are at the center, or near the wall. But statistical studies in peculation theory have lead to speculations that most of the matter is located near the boundary's of these bubbles. Therefore it is unlikely that we are located at the centre.

This of course throws up the spectra that the mass of our universe (although statistically similar with most other space/times) is different to space/time located at the center.

And of course our bubble may have been just one of many (infinite amount?) other false vacuum bubbles that could have formed. Many of those would have quickly collapsed to due to gravity, but many others would have survived the collapse and are now inflating like our bubble.

It may be that this super universe may truly be infinite and within it is a sea of foamy false vacuum bubbles , of which within each is a separate universe.

Oh, as for mass , it must be remembered that expansion energy and gravitational energy can be converted to `mass`.

 

i thought the universe is constantly exspanding as the big bang is still in the explode process and will shrink during the implode process so by the time u calculate the diameter it has changed correct??? anyone?

 

JUST thinking....

The visible horizon of our universe is expanding at the speed of light.
And the universe as a general is expanding (even accelerating!).
This may or may not be because of the `explosion` of the big-bang (and it`s just coasting along..).
The expansion may be a result of a repulsive force (like anti-gravity). And the big-bang just created the space/time frame work in a period of super-inflation (?) into which our universe is expanding into....


anyone?

 

i would say its exspanding into its own death as it will exspand untill the gravitation takes over and pulls it back in faster than it exspanded as it needs a bigger force to pull it in than the original force of exspansion so it will in my thinking be imploding at a greater acceleration or speed than exploding?? do u agree blobrana??

 

I`ve forgotten the link but the recent findings from the WMAP Probe ( looking at the fine structure of the cosmic background radiation) gave results that imply that the universe is 13.7 billion years old and that it WILL EXPAND forever.

My personal view is that the universe is exactly poised on <b>omega = 1</b>....

So it looks like we can forget that nice cyclical bang-crunch-bang-crunch-bang-crunch....etc

Please Register or Log in to view the hidden image!

The universe is <b>accelerating</b> and will expand forever...

Unless theres a big-rip.... (google search)

Whoops found it ( at Microsoft news channel, how apt)
http://www.msnbc.com/news/881685.asp

 

if the universe exspands forever that means the planets will disconnect from there orbits as they wont be held in place with the gravity of other planets??

 

Yes.

Well, that`s what the big-rip proposes, if the strength of the Dark energy grows (exponentially/inversely?) then it will first overcome gravity, then perhaps next the electro-weak force and so on until there is nothing (or everything?) except quark pairs ....


i wonder if that energy would then be converted into splitting the quark pairs ( and forming new quark pairs? that in turn split!)?
A runaway reaction that may even expand (decompress) the extra dimensions and so on...


Hum, looks like a very big bang....

(HEY!, i like this theory!)

Please Register or Log in to view the hidden image!

 

Okay, I'm having some difficulties understanding this here, but I'm appreciating your assistance.

1) Was the mass of today's observable universe (as opposed to the universe as a whole) the mass that was crammed into a volume of space that was Planck length in diameter at Planck time after the Big Bang?

2) If yes, does that mass remain the same today because it is all accounted for as we look further away and backward through time (even as the observable universe grows at the speed of light because of the accelerating expansion of space)?

3) If yes, is it not true that as time (many billions of years) goes on, we will no longer be able to see most of the galaxies (and therefore some of the mass discussed above) because the expansion of the universe will have pushed them forever beyond the event horizon of the visible universe so that we will never see their light again? If this is the case, then the mass of the observable universe seems to decrease over time. So if it will decrease in the future, how do we know that the mass of the observable universe hasn't decreased since shortly after the Big Bang? Sorry if I'm not even making any sense here.

 

Yes to all three...

We can be sure of the original amount of matter/energy created at the BB due to other techniques, such as photon density observed today ,and the relative amounts of lithium and deuterium ( an indicator of proton density), as well simple mathematical tools (to backtrack) and direct observations of the early universe...

Have a look at this related article...(CPT violation,...)
http://www.spacedaily.com/news/physics-03d.html

 

The answer to (1) is, in fact, no - not yes.

The mass-energy of the universe comes from the reheat phase after the decay of the inflation fields at the end of the inflationary epoch. This is about 10^-32 seconds after BB, which is the GUT transition phase. Somebody earlier up the thread mentioned the Planck time in connection with the GUT phase transition - not so. The Planck time is the quantum gravity era.