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spoerl78
2009-Jan-21, 01:04 PM
Hello everybody,

I wonder because it is said that:

1. We can't look further in the past than the Cosmis Micromave Backround
(CMB) [may this maybe extended when we know how to look at neutrinos]
and
2. due to inflation there are parts in the universe where we will never
be able to look at because they are too far away the accelerated expansion of the universe hinders the light to reach us (I think this may also be treated as akind of black hole due to Hawking).

So where is the border of things that we can actually look at?
Does it lie beyond the CMB?

antoniseb
2009-Jan-21, 01:30 PM
So where is the border of things that we can actually look at? Does it lie beyond the CMB?

The CMB represents a time (roughly 380,000 years after the big bang) when the universe cooled enough for plasma to turn into atoms (which were more transparent). We really won't see photons from before this time.

We have observed things other than photons that give us clues about the universe prior to the CMB-time, including isotope abundances in 'primordial material'.

sethoscope
2009-Jan-21, 01:59 PM
I believe what you are refering to in the second part of you statement is the Hubble distance.

An article in the March 2005 Scientific American explained it nicely.

"In expanding space, recession velocity keeps increasing
with distance. Beyond a certain distance, known as the
Hubble distance, it exceeds the speed of light. This is not a
violation of relativity, because recession velocity is caused
not by motion through space but by the expansion of space.

.. . . . .

t h e i de a of se e i ng faster-than-light galaxies may sound
mystical, but it is made possible by changes in the expansion
rate. Imagine a light beam that is farther than the Hubble
distance of 14 billion light-years and trying to travel in our
direction. It is moving toward us at the speed of light with
respect to its local space, but its local space is receding from
us faster than the speed of light. Although the light beam is
traveling toward us at the maximum speed possible, it cannot
keep up with the stretching of space. It is a bit like a child trying
to run the wrong way on a moving sidewalk. Photons at
the Hubble distance are like the Red Queen and Alice, running
as fast as they can just to stay in the same place.
One might conclude that the light beyond the Hubble distance
would never reach us and that its source would be forever
undetectable. But the Hubble distance is not fixed, because
the Hubble constant, on which it depends, changes with
time. In particular, the constant is proportional to the rate of
increase in the distance between two galaxies, divided by that
distance. (Any two galaxies can be used for this calculation.)
In models of the universe that fit the observational data, the
denominator increases faster than the numerator, so the Hubble
constant decreases. In this way, the Hubble distance gets
larger. As it does, light that was initially just outside the Hubble
distance and receding from us can come within the Hubble
distance. The photons then find themselves in a region of
space that is receding slower than the speed of light. Thereafter
they can approach us."

So even if there are photons beyond our reach now, it doesn't mean that they always will be. The Hubble distance is not a fixed distance and grows with time.

I'll try to find the link where you can download the entire article. If you think it helps with your question let me know.

spoerl78
2009-Jan-21, 03:07 PM
I believe what you are refering to in the second part of you statement is the Hubble distance.



Yes. How is it calculated? HUBBLE constant x Age of the universe?



rate. Imagine a light beam that is farther than the Hubble
distance of 14 billion light-years and trying to travel in our
direction.


Is 14 billion years the age of the observable universe plus the Dark Age
before the recombination?



As it does, light that was initially just outside the Hubble
distance and receding from us can come within the Hubble
distance. The photons then find themselves in a region of
space that is receding slower than the speed of light. Thereafter
they can approach us."

So even if there are photons beyond our reach now, it doesn't mean that they always will be. The Hubble distance is not a fixed distance and grows with time.


This would mean that, if the expansion of the universe is slowing down
(is this really the case? I thought it's still accelerating)
would in the end see our whole universe?

At least helped a bit for the second part of the question. Thanks!

sethoscope
2009-Jan-21, 04:00 PM
Here is the link to some additional information on the Hubble distance.

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

This really helped me understand Hubble's Law.

Cougar
2009-Jan-21, 04:03 PM
So where is the border of things that we can actually look at?.... Does it lie beyond the CMB?

Depends on what you mean by "look." If you switch your detector from electromagnetic waves to gravitational waves (assuming they exist and our instruments can detect them one day), you will be able to "see" well past the surface of last scattering.

spoerl78
2009-Jan-21, 04:07 PM
Depends on what you mean by "look." If you switch your detector from electromagnetic waves to gravitational waves (assuming they exist and our instruments can detect them one day), you will be able to "see" well past the surface of last scattering.

Right but it only shifts the problem. Do you think that we will detect
gravitational waves before we would understand how to use neutrinos?

trinitree88
2009-Jan-23, 12:27 PM
Depends on what you mean by "look." If you switch your detector from electromagnetic waves to gravitational waves (assuming they exist and our instruments can detect them one day), you will be able to "see" well past the surface of last scattering.

Cougar. That's interesting. You ought also to be able to see acoustic fluctuations in those gravitational waves and the prompt neutrino fluxes from that era as well, No? pete

spoerl78
2009-Jan-23, 01:20 PM
Cougar. That's interesting. You ought also to be able to see acoustic fluctuations in those gravitational waves and the prompt neutrino fluxes from that era as well, No? pete

Since neutrinos decoupled earlier than photons this should be possible,
and gw even decoupled earlier. We'll get a lot more of information when
we learn how to "look" at them.

But I'm still worried where this border of this is (and even accelerating away from us) that we can't look, in one or the other way?!

speedfreek
2009-Jan-23, 05:02 PM
I wonder because it is said that:

1. We can't look further in the past than the Cosmis Micromave Backround
(CMB) [may this maybe extended when we know how to look at neutrinos]
and
2. due to inflation there are parts in the universe where we will never
be able to look at because they are too far away the accelerated expansion of the universe hinders the light to reach us (I think this may also be treated as akind of black hole due to Hawking).

So where is the border of things that we can actually look at?
Does it lie beyond the CMB?

There are two horizons involved here:

1. The particle horizon, otherwise known as the surface of last scattering. This horizon is brought about by the recession of the coordinates from which the CMBR was emitted and currently lies some 46.5 billion light years away. We have received no CMBR photons so far that were emitted from regions that are currently more distant than 46.5 light years. This is the diameter of our observable universe, it is the limit on the distance we can see back in time. As time goes on we should receive CMBR photons that were originally emitted from regions which are now more than 46.5 billion light years away.

2. The cosmological event horizon, sometimes known as the light horizon. This horizon is brought about by acceleration of the rate of expansion. We can see galaxies that were beyond our Hubble distance when they emitted their light, but we cannot ever see galaxies that were outside of our cosmological event horizon as that is, in effect, the distance where light itself recedes at the speed of light and can make no progress towards us. This horizon is currently around 16 billion light years away, and we will never see any event that happens now, if it happens beyond that distance. It is the limit on the distance we can see through space, but it will be many billions of years before the photons that are now being emitted 16 billion light years away will finally reach us.

spoerl78
2009-Jan-23, 08:56 PM
There are two horizons involved here:

1. The particle horizon, otherwise known as the surface of last scattering. This horizon is brought about by the recession of the coordinates from which the CMBR was emitted and currently lies some 46.5 billion light years away. We have received no CMBR photons so far that were emitted from regions that are currently more distant than 46.5 light years. This is the diameter of our observable universe, it is the limit on the distance we can see back in time. As time goes on we should receive CMBR photons that were originally emitted from regions which are now more than 46.5 billion light years away.

2. The cosmological event horizon, sometimes known as the light horizon. This horizon is brought about by acceleration of the rate of expansion. We can see galaxies that were beyond our Hubble distance when they emitted their light, but we cannot ever see galaxies that were outside of our cosmological event horizon as that is, in effect, the distance where light itself recedes at the speed of light and can make no progress towards us. This horizon is currently around 16 billion light years away, and we will never see any event that happens now, if it happens beyond that distance. It is the limit on the distance we can see through space, but it will be many billions of years before the photons that are now being emitted 16 billion light years away will finally reach us.

to 1.: I thought we could only look back to 13.4 Billion years (Age of the universe (13.7) minus dark age(0.4)). But you say that we can see a universe with a diameter of 46.5 ly, which means that we can see (in one direction) light that is 23.25 y old. I don't get that.

to 2.: Even 16ly is larger than what I expect to be seen (13.4 ly). Will we see this horizon in year 16.000.000.000? Sorry I can't wait that to get the answer :)

Could you conclude your facts! I can't :cry:

speedfreek
2009-Jan-23, 09:13 PM
Have you had a look at the pdf file that sethoscope linked in post #5?

spoerl78
2009-Jan-26, 08:10 AM
Ooops, sorry I forgot. Now I read it, but I'm still confused.
My final question is: Can we see this horizon which behaves
like a BlackHole?

speedfreek
2009-Jan-26, 06:38 PM
Well not really, but we can see the light we receive that was emitted inside the cosmological event horizon. We can never see light that was emitted outside our cosmological event horizon, as the acceleration of the rate of the expansion of the universe is putting more and more distance in between us and those photons.

It's only superficially like a black hole, as the cosmological event horizon moves with the observer - if you were able to instantly transport yourself 1 billion light-years in a certain direction from here, your cosmological event horizon would move with you and you would be able to see 1 billion light-years further in that certain direction than we can here. Of course, you would be able to see 1 billion light-years less in this direction.