# The observable universe

#### BOAS

Hello,

i'm reading through a book called "Discovering The Universe" and in the chapter about cosmology the familiar idea of the observable universe is discussed.

It speaks of the radius of the cosmic particle horizon being ~15 billion light years, which makes perfect sense as light from further away has not had time to reach us yet.

But I have a question. Have we ever seen, or are we actively looking for new stars in the night sky? Am I right in thinking as time goes on, it's possible that light from a never before seen star will reach earth and will effectively 'appear' in the sky.

Are we likely to notice if one does?

Thanks!

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#### phinds

Gold Member
The size of the observable universe is increasing slightly over time so yes, new stars will move into it, BUT ... light from a single star at that distance will be too weak to detect. Google the "Hubble Deep Field" for a discussion of how GALAXIES at fairly close to that distance are detectable, but only with some effort.

#### Chronos

Gold Member
I don't think we will be seeing any light that is older than the universe. The CMB is the effective limit on how far we can see into the universe in EM wavelengths. It is not quite as old as the universe itself. To see any further we would require neutrino or gravity wave detectors. These are a work still in progress.

#### Low-Q

Gold Member
I don't think we will be seeing any light that is older than the universe. The CMB is the effective limit on how far we can see into the universe in EM wavelengths. It is not quite as old as the universe itself. To see any further we would require neutrino or gravity wave detectors. These are a work still in progress.
I assume you mean that we can't see light that is older than the universe as we know it?
I'm not trying to post personal views here, but it is tempting to believe that the universe doesn't expand faster than light, so that what we can observe (if we could observe the most distant object) in fact is the limit of the scope of the universe (?).

Vidar

#### phinds

Gold Member
I'm not trying to post personal views here, but it is tempting to believe that the universe doesn't expand faster than light,
The two parts of that statement are mutually contradictory. The universe DOES expand faster than light so to say otherwise is an unsupportable personal theory.

#### BOAS

The two parts of that statement are mutually contradictory. The universe DOES expand faster than light so to say otherwise is an unsupportable personal theory.
Is it still thought to be expanding so fast?

I know that rapid expansion is part of the accepted cosmological model for the early universe. Hence why the diameter of the observable universe is ~90 billion light years.

#### Chronos

Gold Member
The 90 billion light years thing is a source of confusion. It is true the observable universe is believed to be this large NOW, but, we cannot and will never see the universe as it is NOW. We see the universe at the age it was when photons we detect were emitted. For example, the CMB, the most ancient photon source possible to detect, was a mere ~40 million light years distant when the photons we now detect were emitted. Due to expansion, it took those photon 13.7 billion years to reach us. In the mean time, the CMB has receded to 90 billion light years, also due to expansion.

#### phinds

Gold Member
Is it still thought to be expanding so fast?
No, it is not THOUGHT to be, it is known to be. Objects at the edge of our observable universe are receding from us at about 3c.

#### BOAS

No, it is not THOUGHT to be, it is known to be. Objects at the edge of our observable universe are receding from us at about 3c.
Cool - I didn't know that.

#### Low-Q

Gold Member
No, it is not THOUGHT to be, it is known to be. Objects at the edge of our observable universe are receding from us at about 3c.
So objects CAN travel faster than light? When was this discovered? Because yesterday I could read that objects cannot travel as fast as light does.

Vidar

#### phinds

Gold Member
So objects CAN travel faster than light? When was this discovered? Because yesterday I could read that objects cannot travel as fast as light does.

Vidar
No, objects cannot travel faster than light and no one has said they can or do. "Receding" is not the same thing. Google "metric expansion" for an explanation.

#### George Jones

Staff Emeritus
Gold Member
Adding to the confusion :tongue: :
As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.
In more detail:
I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55.

Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

Code:
O                                B        A        C
*                                *        *        *

*                    *     *     *
O                    B     A     C
The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, $t_e$. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, $t$. Notice that space has "expanded" between times $t_e$ and $t$.

Suppose that at time $t_e$: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time $t$, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because A's recession speed at time $t_e$ is greater than c, the pulse fired towards us has actually moved away from us between times $t_e$ and $t$.

Now, suppose that the distance from us to galaxy B at time $t$ is the same as the distance to galaxy C at time $t_e$. Even though the distances are the same, the recession speed of B at time $t$ is less than than the recession speed of C at time $t_e$ because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times $t_e$ and $t$.

Since A's recession speed at time $t_e$ is greater than c, and galaxy C is farther than A, galaxy C's recession speed at time $t_e$ also is greater than c. If, however, the Hubble constant decreases enough between times $t_e$ and $t$, then B's recession speed at time $t$ can be less than c. If this is the case, then at time $t$ (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times $t_e$ and $t$.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time $t_e$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time $t$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

#### Low-Q

Gold Member
No, objects cannot travel faster than light and no one has said they can or do. "Receding" is not the same thing. Google "metric expansion" for an explanation.
Thanks :-)

Vidar

#### Philosophaie

The size of the observable universe is increasing slightly over time so yes, new stars will move into it, BUT ... light from a single star at that distance will be too weak to detect. Google the "Hubble Deep Field" for a discussion of how GALAXIES at fairly close to that distance are detectable, but only with some effort.
Light would be Red-Shifted if the source is moving away. The universe is expanding and a new star just forming at the edge of the universe would probably be moving away also.

#### phinds

Gold Member
Light would be Red-Shifted if the source is moving away. The universe is expanding and a new star just forming at the edge of the universe would probably be moving away also.
Yes, but the fact that is moving away doesn't mean its light wouldn't reach us, it just means that as you say it would be red-shifted. It also would be incredibly faint, both of which combine to mean that it would not likely be detectable. "Not detectable" however, is not the same as "does not exist".

#### Chronos

Gold Member
But, such a star would not be 'new' in the sense of expansion overtaking a hitherto unobservable region of space. Since all photons originate inside the CMB, the region of the universe containing that star has always been observable. Remember too, that all events we observe in the remote universe are also time dilated. So the 'birth' of an object at high redshift would appear to be in slow motion as observed from earth. This why distant SN1a take longer to peak in luminosity than low z supernova.

#### KyleScott

We won't detect any new stars or galaxies at that distance not only because the stars would be too faint, but because we'd be waiting for a very long time to detect much of anything. For example, if an undetected galaxy is relatively close to the edge of the visible universe--let's say 1 million LY--we wouldn't be able to detect it for a million years. The timescale is simply so great that we don't look for changes in things like that.

#### phinds

Gold Member
For example, if an undetected galaxy is relatively close to the edge of the visible universe--let's say 1 million LY--we wouldn't be able to detect it for a million years. .

No, if it were 1 million LY from the edge of the observable universe, the light would not arrive here for something like 49 BILLION light years.

#### KyleScott

No, if it were 1 million LY from the edge of the observable universe, the light would not arrive here for something like 49 BILLION light years.
Allow me to clarify: a million years from now.

And what do you mean that it would arrive in 49 billion LIGHT-years? lol

#### phinds

Gold Member
Allow me to clarify: a million years from now.

And what do you mean that it would arrive in 49 billion LIGHT-years? lol
Yep, I blew that one. I meant 47 billion years, not LY. That's how long it will take a new photon from the edge of the observable universe to reach the Earth.

#### Chronos

Gold Member
Actually, light emitted TODAY from a body a million light years in the foreground of the CMB will NEVER get here. z~1.7 is the maximum distance from which a photon emitted NOW is capable of reaching earth. See Lineweaver and Davis - Expanding Confusion.

#### Fermifaq

was reading up on this the other day and found this useful

Comoving distance
http://en.wikipedia.org/wiki/Comoving_distance

Hubble volume
http://en.wikipedia.org/wiki/Hubble_volume

In cosmology, a Hubble volume, or Hubble sphere, is a spherical region of the Universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the Universe

"The Real Universe" --'Is 250 Times Bigger than the Visible Hubble Volume'
http://www.dailygalaxy.com/my_weblog/2013/02/the-real-universe-is-250-times-bigger-than-the-visible-hubble-volume-todays-most-popular-1.html

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

As the universe gets older, and our light cone gets larger we will start to see a bit more of the universe that is currently hidden from us, but some stuff will permanently remain out of view.

There is also something called the 'lookback time'
http://en.wikipedia.org/wiki/Redshift#Highest_redshifts

Last edited:

#### Chronos

Gold Member
I assume you had a point, unfortunately I didn't get it.

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