Ever Expanding Cosmology

[darrellx (Darrell)]

Hi all

This seems to be the place to ask the deep questions stemming from magazine articles. So I have one from the recent "Astronomy" magazine, August 2008 issue - the "Special 35th Anniversary Issue".

The article "Is there an end to Cosmology" begins on page 28.

The thrust of the article is that the universe is expanding, and eventually all other galaxies will be out of sight.

The current belief is that only galaxies within 3million light years will remain inside our horizon of view while all else will move further away as the universe expands. Essentially becoming "unviewable". The only two large galaxies within this range are the Milky Way and Andromeda.

I am fine with this - so far.

So, the light we see from galaxies now, was emitted a very long time ago. "What we see does not represent their current state."
"We will never be able to study the evolution of a galaxy beyond some finite age in its own frame of reference. The more distant a galaxy is, the earlier its image will freeze, ...."

Here is where I have the dilema.

If,as the article suggests, those distant galaxies become unviewable and freeze in time (to us), due to their distance, and the further a galaxy is from us, the earlier they "freeze", how do we know that the process isn't already well underway?

Could it be that the 3 million light year horizon mentioned above has started further out. If the more distant galaxies "disappear" first, maybe galaxies beyond 13.?? billion light years have simply disappeared, making us THINK that the universe started 13billion years ago. Just as the author suggests future cosmologist will only think the one galaxy is all that exists because that is all they can see.

Darrell

[avandonk]

Correct we can only 'see' back in time that is within a light cone. We can never know the infinite. We can only infer by what we see now.

As for the age of the Universe it is about 13.7 billion years.

Consider this and this is Olbers Paradox and mentioned by Edgar Allan Poe. If the Universe was truly infinite and was always there then the night sky would be as bright as any sun. No matter where you looked there would be a star! Why it is not is due to the finite speed of light and the fact that the Universe had a beginning.

Sorry this is the best I can do for now.

Bert

[sjastro]

Originally Posted by darrellx

Could it be that the 3 million light year horizon mentioned above has started further out. If the more distant galaxies "disappear" first, maybe galaxies beyond 13.?? billion light years have simply disappeared, making us THINK that the universe started 13billion years ago. Just as the author suggests future cosmologist will only think the one galaxy is all that exists because that is all they can see.

Darrell

The age of the Universe (13.7 billion +/- 0.13 billion years) is determined by the fluctuations in the Cosmic Radiation Background. The Cosmic Radiation Background is composed of extreme red shifted photons from the first atoms created about 400,000 years after the BB. Since atoms preceded galaxies in cosmic evolution we know the oldest galaxies cannot be much more than 13 billion years old.

Regards

Steven

[Jeff]

Hi Darrell,

Just thinking this through, the furthest Messier galaxy in the Virgo galaxy cluster (M58) is about 68 Mly away, and has a redshift of about 1,500 km/sec (1/200, or approx 0.5% of the speed of light).

Even if the the rate of cosmological expansion continues is increase (as many are now proposing), I would expect new light from these galaxies to still be viewable our neighbourhood in the distant future (albeit with more cosmological redshift). By my thinking only cosmological expansion at/above the speed of light would prevent this (and I'd like to see that).


Have not read that article, but the quoted phrases sound wacko to me.
Could it be a cunning plan to sell magazine subscriptions?
Any chance of scanning and posting a couple of jpg's of the article pages?

Cheers,
Jeff

[sjastro]

Originally Posted by Jeff

Even if the the rate of cosmological expansion continues is increase (as many are now proposing), I would expect new light from these galaxies to still be viewable our neighbourhood in the distant future (albeit with more cosmological redshift). By my thinking only cosmological expansion at/above the speed of light would prevent this (and I'd like to see that).
Cheers,
Jeff

The earliest galaxies are red shifted into the infrared spectrum that they are invisible in optical instruments.

Since the universe is permeated with a Cosmic Infrared Background (as opposed to the BB remnant Cosmic Microwave Background), detecting these galaxies with IR telescopes is extremely difficult.

It's analogous to the difficulties of imaging a very faint galaxy in a light polluted sky.

Regards

Steven

[Jeff]

Originally Posted by sjastro

The earliest galaxies are red shifted into the infrared spectrum that they are invisible in optical instruments.

Since the universe is permeated with a Cosmic Infrared Background (as opposed to the BB remnant Cosmic Microwave Background), detecting these galaxies with IR telescopes is extremely difficult.

It's analogous to the difficulties of imaging a very faint galaxy in a light polluted sky.

Regards

Steven

Have had a peek at the article (attached).

I think the author is saying that as the expansion of the cosmos is thought to be accelerating, a point will be reached in the distant future (another 143 billion years) when light from galaxies outside our local group will no longer reach us. Distant galaxies will get to a point beyond redshift ... where the cumulative cosmological expansion between them and us will exceed the speed of light so that new light being emitted from these galaxies can no longer reach us.

As I understand things, their images will not "freeze" (as in a still frame), but would progressively redshift until no new lightwaves/photons can catch us.

Looks like we haven't much time ...we need to band together and convince our spouses/politicians that investments in astronomy equipment need to be significantly increased, and fast !!!

[sjastro]

Thanks for this interesting article Jeff.

There are couple of questionable issues.

First of all the author's assertion that the distant galaxies being out of sight is "similiar to watching a friend cross a black holes event horizon" is wrong. The friend would be observed to be trapped on the event hoizon of a black hole. This is an example of gravitational red shift, not Doppler red shift which is the basis for the article.

Secondly "Within a finite time, the accelerated expansion of space moves any distant galaxy away from us that exceeds even that of light" is frankly misleading as it implies the galaxy itself is travelling faster than light.

Cosmologists use a comoving or expanding coordinate system in which each galaxy in the universe is in a fixed postion despite the universe expanding.
If the velocity of a distant galaxy in a comoving coordinate system is calculated, then yes, it can exceed the speed of light.

Basic physics and Special Relativity define velocity in a local coordinate system with a fixed origin and axes. When the comoving coordinate system
is converted to a local coordinate system no galaxy travels faster than light.

Do galaxies actually "freeze"? I wonder.

Regards

Steven

[drmorbius (Randall)]

Originally Posted by Jeff

Looks like we haven't much time ...we need to band together and convince our spouses/politicians that investments in astronomy equipment need to be significantly increased, and fast !!!

I'll let you know how I go...