Is 5 billion ly really 5 billion ly?

[nub340]

Here is something that has always baffled me:

When you look up at a star that's 5 billion light years away, most people would agree that you are looking at the light that left the star 5 billion years ago and that it took the light 5 billion years to finally get here. What confuses me is how that plays nicely with inflation, the fact that the universe is getting bigger & bigger faster & faster. Doesn't inflation say that 5 billion years ago the universe was a whole lot smaller and wouldn't that mean 5 billion years ago that star was actually a whole lot closer to us than 5 billion light years and if so wouldn't that light have reached us a long time ago?

 

[Shpaget]

Not exactly.
It would mean that that particular star is more than 5 b ly away "now", because it was 5 b ly away 5 billion years ago when it produced the light that we are observing today.

(I am right, right?)

 

[origin]

Here is something that has always baffled me:

When you look up at a star that's 5 billion light years away, most people would agree that you are looking at the light that left the star 5 billion years ago and that it took the light 5 billion years to finally get here. What confuses me is how that plays nicely with inflation, the fact that the universe is getting bigger & bigger faster & faster. Doesn't inflation say that 5 billion years ago the universe was a whole lot smaller and wouldn't that mean 5 billion years ago that star was actually a whole lot closer to us than 5 billion light years and if so wouldn't that light have reached us a long time ago?

Couple of things.

The term inflation as it applies to the universe is specifically about an event that occurred within the first second of the universes creation. The term to describe the increase in the size of the universe today is expansion.

To your question the answer is somewhat complicated - what a shock, huh? The bottom line is that if the light took 5 billion years to reach earth then the light traveled 5 billion light years. However that does not mean that the star was 5 billion light years away when the light left the star. Lets use some fake numbers (becasue I don't feel like looking up or calculating the real numbers [speedfreak probably knows them off the top of his head!]) but the point will be valid. Lets assume a star (or galaxy, really a star is to dim to be seen at 5 billion ly) is 4 billion light years away. The light leaves the star and heads towards earth. Meanwhile the distance between earth and the star is increasing as the space between them expands so that when the light reaches earth the distance covered is 5 billion light years. Now of course all of this time the universe has continued to expand so that the star is actually 6 billion LY away from earth when the 5 billion year old light reaches earth.

 

[SpeedFreek]

That was a pretty good guess there, origin!

A light-travel time of 5 billion years is represented by galaxies with a redshift of around z=0.5

Galaxies with a light-travel time of 5 billion years were actually only around 4 billion light-years away when they emitted the light we are now seeing.

The expansion of the universe was constantly putting more distance in between that galaxy and the Milky-Way, during the time that the light was travelling for.

As the light finally reaches us, that galaxy is now around 6 billion light-years away.

The redshift factor, z, is indicative of the scale factor of the universe when used in the form 1+z. So, that means that the universe is now around 1.5 times larger than it was when the light was emitted. The universe was only 2/3rds of its present size when that light was emitted, so that galaxy was only 2/3rds of the distance away at the time of emission, when compared to where it would be today.

For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 0.500

* It is now 13.665 Gyr since the Big Bang.
* The age at redshift z was 8.647 Gyr.
* The light travel time was 5.019 Gyr.
* The comoving radial distance, which goes into Hubble's law, is 1881.7 Mpc or 6.137 Gly.
* The comoving volume within redshift z is 27.909 Gpc^3.
* The angular size distance DA is 1254.5 Mpc or 4.0916 Gly.
* This gives a scale of 6.082 kpc/".
* The luminosity distance DL is 2822.6 Mpc or 9.206 Gly.

 

[Shpaget]

Meanwhile the distance between earth and the star is increasing as the space between them expands so that when the light reaches earth the distance covered is 5 billion light years. Now of course all of this time the universe has continued to expand so that the star is actually 6 billion LY away from earth when the 5 billion year old light reaches earth.

That's what I said. :?

 

[SpeedFreek]

Meanwhile the distance between earth and the star is increasing as the space between them expands so that when the light reaches earth the distance covered is 5 billion light years. Now of course all of this time the universe has continued to expand so that the star is actually 6 billion LY away from earth when the 5 billion year old light reaches earth.

That's what I said. :?

Except that the galaxy was only 4 billion light-years away when it emitted the light we are now seeing. You had the right idea though.

 

[Shpaget]

aaarrrgggghh
I quoted the wrong part of the reply.
I meant to quote origin:

Now of course all of this time the universe has continued to expand so that the star is actually 6 billion LY away from earth when the 5 billion year old light reaches earth.

Sorry about that.

The only question is whether the calculations on the distances only measure the distance the light traveled from the point of it's origin or do they include the speed the source is moving away.
If it's the first case, than the stars, and galaxies, are actually further away than measured, if the latter is true, than you get the correct distance.

 

[MeteorWayne]

I knew you could reply better and with more details than I SpeedFreak, so I kept my mouth shut

 

[origin]

That was a pretty good guess there, origin!

And once again the blind squirel anology can be applied to me. Hooray :lol:

 

[xXTheOneRavenXx]

lol, Speedfreak being a show off again. The Holy Oracle of the universe. Einstein eat your heart out. Nah, just kidding. But I have to say, your pretty damn good at the number crunching.

 

[SpeedFreek]

I don't number crunch, unfortunately. I simply plug the numbers into a calculator that does all the hard work for me.

http://www.astro.ucla.edu/~wright/CosmoCalc.html

http://www.wolframalpha.com/examples/Astrophysics.html

http://icosmos.co.uk/

There! The cheater is revealed! :shock:

Note: The real trick, of course, is knowing how to use the calculator properly!

 

[R1]

Ok, here is a more drastic situation question:
How far was the CMBR seen today, really, when it was emitted ?



And though it may be impossible to ever see past it,
Whatever IS just past it,
Must have been closer to the potential earth ,right?
Closer to us than the Voyagers are?
Or how close?
Eventually a point would be reached that such point would have had to be adjacent to us, correct?

One thing that would blow this out of proportion could be that inflation expanded the universe
much faster than the speed of light.
Wasn't that indeed the case?

As the drastic question continues it could make me wonder about information and causality.
If the universe's inflation exceeded the speed of light, then information about the beginning would
actually not exist until the end, if ever, correct?

If the universe during inflation had nothing (no matter!), then all that nothingness expanded
faster than the speed of light and essentially became like a third twin (triplet, actually), who
exceeded the speed of light, right ?

:?

 

[R1]

Things can get confusing then.

If inflation was faster than the sped of light, then who and how determines the duration of inflation?
Those precious FTL seconds can practically and easily last 5 billion years and then some, can they not?
(I think they can, but its duration would result in negative time, wouldn't it ?)

Did not the nothingness which became inflated faster than light -- which in fact became everything--actually then become the past of the bang ? It would seem so, if by the time the bang sent or received information to or from
the potential universe, things were already in place for matter to exist.

 

[SpeedFreek]

Ok, here is a more drastic situation question:
How far was the CMBR seen today, really, when it was emitted ?

The CMBR photons we detect today were originally emitted a mere 42 million light-years away, approximately 370,000 years after the Big-Bang.

http://www.wolframalpha.com/input/?i=redshift+z%3D1089

And though it may be impossible to ever see past it,
Whatever IS just past it,
Must have been closer to the potential earth ,right?
Closer to us than the Voyagers are?
Or how close?
Eventually a point would be reached that such point would have had to be adjacent to us, correct?

Yes, that is correct, the further back you consider, the closer it was originally, all the way back to when considering that all coordinates were right next to each other.

One thing that would blow this out of proportion could be that inflation expanded the universe
much faster than the speed of light.
Wasn't that indeed the case?

Yes that's right. But it is worth noting that the universe has always expanded faster than light - the question is how far apart are the coordinates that are separating at c.

Right now, coordinates approximately 14 billion light-years away have recession velocities of c, and anything further away is receding superluminally. The edge of the observable universe (the particle horizon or surface of last scattering) is over 3 times that distance away right now as is receding at around 3 times the speed of light.

But during the inflationary epoch, coordinates down at the Planck scale were separating faster than light - that is the difference between "normal" expansion and superluminal inflation. During inflation, everywhere is expanding faster than light!

As the drastic question continues it could make me wonder about information and causality.
If the universe's inflation exceeded the speed of light, then information about the beginning would
actually not exist until the end, if ever, correct?

The information existed for a while before the inflationary epoch, and it was during this time that the conditions across the universe were able to find equilibrium. Then, during inflation, that information was stretched out across vast distances and only after that are we left with an observable universe that is causally disconnected from the rest of it.

If the universe during inflation had nothing (no matter!), then all that nothingness expanded
faster than the speed of light and essentially became like a third twin (triplet, actually), who
exceeded the speed of light, right ?

I'm not sure what you mean, here.

 

[R1]

I get confused with the superluminal now.
I have heard of the twin paradox, or something like that.
One twin stays on earth, and the other one takes a brief trip at c.

What I wonder now is suppose we have 3 triplets just before the big nothingness went bang (so to speak).
The first triplet remains static, the second one I don't know what he does,
maybe he takes his usual brief trip at c, but the third one takes a brief trip with inflation at FTL speeds !

The third triplet in this example would be going into negative time, would he not?
I think he could actually fly around the inflating nothingness and practiacally cause
changes that could be masured by triplet #1 as having actually occurred 'in the past.

I don't know how or what role time plays in quantum fluctuations, but yet iirc energy can somehow
be 'borrowed' from the future, and used in the present (which is the future's past).

 

[SpeedFreek]

The third triplet in this example would be going into negative time, would he not?

We cannot have negative time, otherwise we end up having effects before they are caused.

Special Relativity holds that no object with mass can accelerate to c or beyond, but SR does not apply here as nothing is travelling through space faster than light. Those distant galaxies aren't moving faster than light. They are not overtaking photons. They are pretty much at rest in relation to their surroundings, just like we are, over here in the Milky Way. We are apparently receding from them faster than light too, from their point of view in those distant galaxies!

It is a difficult concept to understand, as it is counter-intuitive, but a simple way to model the situation is to say that space is expanding in every direction, putting more distance in between us and those distant galaxies. Nothing is moving through that space faster than light, but the space itself is increasing in size in such a way that the distance to those galaxies is increasing.

The further away a galaxy is, the faster it apparently recedes, but all galaxies are essentially at rest and it is the space around them that increases in size. This would throw up apparent recession speeds that were faster than light beyond a certain distance, but nothing is actually moving faster than a photon as even photons get dragged along for the ride by the expansion of the universe. This is why we think the Cosmic Microwave Background Radiation we measure today, which was released nearly 13.7 billion years ago, was originally released only 42 million light-years away!

There is an apparent time-dilation in an expanding universe, but time would stop at redshift=infinity (i.e. the Big-Bang), and would never run backwards.

 

[R1]

I think I just remembered that the earth has been slowing down.
I think the days around the dinosaur age were much shorter.
This makes me wonder how many days was a year back then? And did a year last the same amount of time?

 

[MeteorWayne]

Yes days were shorter, but I'm not sure of the exact time. The year was the same amount of time, but of course, since the days were shorter, there would have been more days in a year.

 

[R1]

Thanks; then it seems safe to use light-year measurements, since the earth years were the same
amount of time. I was beginning to wonder if earth years over long periods of time were
inconsistent durations, but they're not.

 

[MeteorWayne]

Actually, the current definition of a light year is a specific distance, no longer directly derived from an earth year...but since that changes very little for all intents and purposes they are the same.

 

[ZenGalacticore]

I don't recall reading that Earth days were all that significantly shorter just a 100 million to 150 million years ago. Billions of years ago they were, but hardly significantly shorter as recent as the age of the dinosaurs.

My two cents.

 

[SpeedFreek]

Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit

620 million years ago, a day was around 22 hours long.

 

[ZenGalacticore]

Thus just 100 million years ago, in the heyday of the dinosaurs, I would guess the day was around 23.60 hours, or thereabouts.