A Question About the Size of the Universe...

[JimBrown257]

Hello, I am a long-time fan of Mr. Randi but new to posting on these forums. In fact, it's my first time so please be gentle.

Anyways, I have a question that some of you critical thinkers might be able to help me with:

If the universe is about 156 billion light-years across (NASA.com), would it not have to have been inflating at more than the speed of light since the Big Bang around 13.7 billion years ago?

Can anyone explain this to me?

Thanks!

-JB

 

[Undesired Walrus]

156? You sure?

 

[Dancing David]

Welcome, there are no stupid questions, only stupid answers. Here is mine.

There was this theoretical time of Inflation (cosmology)^WP, when the universe expanded very very rapidly. The universe is very large for that reason. And even stranger it may have been larger than a singularity at the time of the Big Bang.

 

[Trent Wray]

Does the idea that the universe is expanding intrinsically have any bearing on this?

 

[nota]

nobody really knows the exact size [except it is very very very big]
or even if the universe has a size or shape or center or edge

we do know how far we can see currently
and that is tied to how old the stuff we can see is when we see it

biggest bit of the size question that is unknown
is the rate of expansion/inflation in the first bits of time post big bang
and if there was any/how much slow down post that time and for how long
AND when the current expansion [dark energy driven] started

we do not know how big the post big bang inflation was
or if there was a slow down caused by gravity post inflation
and when the [dark energy] started a second phase expansion

 

[Towlie]

If the universe is about 156 billion light-years across (NASA.com)...

Nasa.com redirects to http://hus.parkingspa.com, which is a site that sells parental control software. I did manage to track down the page at nasa.gov where you got that 156 billion figure, and it includes an extensive answer to your question. Did you read it?

Dr. Neil Comish and colleagues at Montana State University have done a preliminary calculation on how much the Universe has expanded since the first radiation around 13+ billion years was released and is now just being received on Earth. But because the Universe has expanded tremendously since the Big Bang, the distance that light has been traveling continues to increase during the 13+ billion year timeframe. The source of the primordial light leaving the very early Universe from any point (say, the first star), has thus itself moved much farther than the time-travel distance. Their model leads to an astounding number - 78 billion light years to the edge of today's Universe and a possible diameter of a spherical Universe of 156 billion l.y. (so large both because of the effects of spatial expansion and acceleration rates since about 6 billion years ago. One might conclude that this requires different values for the speed of light in the past, which might seem to violate the General and Special Laws of Relativity, but the astronomers point out that this speed remains constant while the distance a photon released at the beginning of the Universe must travel is what is increasing. This estimated diameter and other figures for size are obviously still controversial but the "true" size should be increasingly refined in coming years.

 

[Phaethon]

I got a very good answer to this question in a class a few years back, but rather than try to explain it myself (which would probably end up poorly!) I just did a quick Google search and turned up with this from Cornell's website:

In English, the two observers cannot give a definition of "inertial" that works in both places. They are separated by curved or expanding spacetime, which is not well-described by either observer's inertial frames.

In the example of the universe, for instance, objects recede from you not because they are in motion, but because space is being created between you and the object. The universe can create as much space as it likes, even so much that it appears that distant objects are travelling faster than c. So long as objects don't reach c relative to any local observer, to whom the expansion is negligible, this is perfectly okay with Einstein.

Not the simplest explanation available, but I trust that if you posted this thread you have some understanding of general relativity.

 

[Tubbythin]

Hello, I am a long-time fan of Mr. Randi but new to posting on these forums. In fact, it's my first time so please be gentle.

Anyways, I have a question that some of you critical thinkers might be able to help me with:

If the universe is about 156 billion light-years across (NASA.com), would it not have to have been inflating at more than the speed of light since the Big Bang around 13.7 billion years ago?

Can anyone explain this to me?

Thanks!

-JB

Ned Wright is always good for this kind of thing (though the numbers differ).

 

[edd]

Hello, I am a long-time fan of Mr. Randi but new to posting on these forums. In fact, it's my first time so please be gentle.

Anyways, I have a question that some of you critical thinkers might be able to help me with:

If the universe is about 156 billion light-years across (NASA.com), would it not have to have been inflating at more than the speed of light since the Big Bang around 13.7 billion years ago?

Can anyone explain this to me?

Thanks!

-JB

It isn't. Best I can tell, someone's doubled a radius to a diameter, and someone else doubled it again thinking it was still a radius.

It's 46 billion light years (comoving distance) from us to the edge of the observable universe. It might turn out the universe is smaller than that if it wraps back on itself, and some physicists went looking for that and got a lower limit slightly less than that. If it were a shade under 40 billion, and you doubled that to 80 billion to get a diameter and someone else did the same thing again then you might get where that 156 figure came from.

At least that's my best guess for where that number came from.

http://en.wikipedia.org/wiki/Observable_universe agrees with me.

 

[shadron]

I've read that some of the numbers involving the inflation are that it commenced at 10^-35 seconds after the start, and that it lasted for 10^-32 seconds. In that time it expanded by a factor of between 10^29 to 10^50 times. Yes, space was expanding much, much faster than the speed of light, but the energy/matter was not itself moving.

The hints are that the expansion had to do with a phase change (analogous to, but not the same as, when water changes to ice) in the universe when the strong nuclear force became separate from the still combined weak and electromagnetic forces. The gravitational force separated itself before the expansion started.

These are somewhat tangential to your request; perhaps it will expand to a elevated level response on what and how inflation happened, as is currently known.

 

[JimBrown257]

Thanks everyone!

Towlie, that page you linked to was very helpful; I had not seen that. I has actually found that 156 bly number on an episode of NOVA at pbs.org; I typed NASA without thinking

I asked someone about it at work today and he suggested reading Hawkins Brief History of Time and Singh's Big Bang. Can anyone else recommend any good books on this subject?

 

[Dancing David]

Um, if you like to read pop-science I really recommend Inflation by Guth, there are some great general histories of cosmology as well, like The Whole Shebang by Ferris and I second the Ned Wright Cosmology Tutorial

 

[readme.txt]

I think the size of the Universe is unknown, but the size of the observable universe is supposed to be about 78 billion ly.

From Wikipedia : http://en.wikipedia.org/wiki/Observable_universe

78 billion light-years. This is a lower bound for the diameter of the whole Universe, based on the estimated current distance between points that we can see on opposite sides of the cosmic microwave background radiation (CMBR).

[snip]

156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter, the doubled figure is incorrect. This figure was very widely reported.

 

[skip]

Not to derail too much here, but for some of you guys that know what you’re talking about the OP made me think of a question.

Is it possible that there are other universes beyond the edge of our universe?

It wasn’t that long ago that the milky way was the universe. Can anyone imagine a time when we can see beyond the edge of our universe and see another?


.

 

[blutoski]

One thing that took me awhile to notice is that the diameter of the universe exceeds what we would expect if the perimeter was expanding slower than the speed of light

eg: an 80 billion ly size means a 40 billion ly diameter, which light takes 40 billion years to traverse. So, how can the universe be 12-23 billion years old?

Evidently, space can expand faster than the speed of light, whereas light can only move through space at a constant speed.

In other words: there's no doubt that space can expand faster than the speed of light. But nothing can traverse through space itself faster than the speed of light.

 

[readme.txt]

Not to derail too much here, but for some of you guys that know what you’re talking about the OP made me think of a question.

Is it possible that there are other universes beyond the edge of our universe?

It wasn’t that long ago that the milky way was the universe. Can anyone imagine a time when we can see beyond the edge of our universe and see another?


.

Dunno. If there are multiple universes beyond the "edge" of this universe and what if they all expand, what will happen when they collide?

 

[sol invictus]

One thing that took me awhile to notice is that the diameter of the universe exceeds what we would expect if the perimeter was expanding slower than the speed of light

eg: an 80 billion ly size means a 40 billion ly diameter, which light takes 40 billion years to traverse. So, how can the universe be 12-23 billion years old?

Evidently, space can expand faster than the speed of light, whereas light can only move through space at a constant speed.

In other words: there's no doubt that space can expand faster than the speed of light. But nothing can traverse through space itself faster than the speed of light.

Space can certainly "expand faster than light"; there's no theory or reason to think it can't. Lots of velocities can be faster than light - the speed of a shadow cast on some surface, the relative velocity of two fast objects moving in opposite directions, the intersection point of the blades of a huge pair of scissors, etc.

As for the "radius of the universe", that means the radius of the part we can currently observe, so there can certainly be more beyond the edge (in fact anything else would be extremely surprising and bizarre). And in general the radius is really not a very well-defined concept - the problem is that there isn't a unique definition of distance in a curved spacetime, and different reasonable definitions will give different answers.

 

[Ziggurat]

It wasn’t that long ago that the milky way was the universe. Can anyone imagine a time when we can see beyond the edge of our universe and see another?

If we can see it, it's in our universe. I suspect what you're asking is whether we will ever be able to see much further away than we currently can. The answer to that is no^*. The farther away you look, the farther back in time you look. When you look back far enough, there aren't any stars, because they haven't formed yet. Look even farther, and you'll run up against the point at which the universe transitioned from opaque to transparent. We can already see that transition, it's called the Cosmic Microwave Background (CMB). We will never be able to see to an earlier time than that^**.

^* Although we will not be able to see anything farther than what we currently see, there is significant room for improvement in how well we can see the early universe. Many objects which are closer than the CMB cannot currently be resolved, but may be in the future.

^** At least, not with light.

 

[Towlie]

Is it possible that there are other universes beyond the edge of our universe?

What makes you think the Universe has an edge?

 

[edd]

If we can see it, it's in our universe. I suspect what you're asking is whether we will ever be able to see much further away than we currently can. The answer to that is no^*. The farther away you look, the farther back in time you look. When you look back far enough, there aren't any stars, because they haven't formed yet. Look even farther, and you'll run up against the point at which the universe transitioned from opaque to transparent. We can already see that transition, it's called the Cosmic Microwave Background (CMB). We will never be able to see to an earlier time than that^**.

^* Although we will not be able to see anything farther than what we currently see, there is significant room for improvement in how well we can see the early universe. Many objects which are closer than the CMB cannot currently be resolved, but may be in the future.

^** At least, not with light.

That's not exactly right. The CMB is not static. Every day we see a little bit deeper. It's sufficiently huge scales involved that we don't see the CMB evolve with time - we're shaving away at it very very slowly, but in principle we are seeing that little bit further every day - a few light years (the light-day it travels is now expanded up to be a thousand times larger, and a thousand days is a few years).

edit: I see you're right that we won't see an earlier time than that, relative to the start, but we will see a further distance as we move further into the future.