Can we trace present day galaxies 6 to 7 billions year back?

[ErnieM]

Using the world's largest camera (570 megapixels) developed by scientists at the Fermi Lab, the Dark Energy Survey beginning next year (http://en.mercopress.com/2011/08/25/wor ... orth-chile) will look back in time (6 to 7 billions years ago) and take pictures of how 300 galaxies (more or less) look like then. On the basis that space or the universe expanded from the big bang and such expansion not only continues but the rate of expansion is accelerating as galaxies move farther apart over time.
The universe is estimated to be 13.7 billion year old. The Milky Way is 13.6 billions years old (http://www.universetoday.com/21822/age- ... milky-way/) so we can assume that other galaxies and galaxy clusters in today's visible universe were also formed at the same time.
This survey is the closest mankind can achieve time travel. What is the possibility that any of all of the 300 or so galaxies showing on this survey be traceable to those we see today? Would it not be wonderful indeed if we can see how our own Milky Way looked like 6 to 7 billion years ago?

[Chris Peterson]

This survey is the closest mankind can achieve time travel. What is the possibility that any of all of the 300 or so galaxies showing on this survey be traceable to those we see today? Would it not be wonderful indeed if we can see how our own Milky Way looked like 6 to 7 billion years ago?

It doesn't work that way. The farther away an object, the farther back in time we see it. We don't see how nearby things looked in the distant past. We only get to see one version of everything.

[ErnieM]

Chris wrote:

The farther away an object, the farther back in time we see it.

Is this the same as saying that distant objects will always be red shifted, never blue shifted?

[bystander]

Is this the same as saying that distant objects will always be red shifted, never blue shifted?

No, objects are red shifted because they are moving away from us (expansion).

[Ann]

ErnieM wrote:
Is this the same as saying that distant objects will always be red shifted, never blue shifted?

Distant objects are seen to move in realtion to our position here on Earth for two reasons:

1) They move because of their own, specific motion.

2) They move because they are part of the expanding "fabric of space", which carries them away from us.

Click to view full size image

M90, a blue-shifted galaxy in the Virgo Cluster. Credit:Canada-France-Hawaii Telescope/Coelum

When it comes to the relatively nearby Virgo Cluster, which is ≈60 million light-years away, the expansion of the universe is enough to make sure that the great majority of galaxies in that cluster have their light redshifted. But because of the large intrinsic motions and velocities within that cluster, a small number of galaxies in the Virgo Cluster are in fact blueshifted. This means that the net motion of the blueshifted galaxies is toward us.

The blueshifted galaxies of the Virgo Cluster are not likely to keep moving in our direction. The gravity of the Virgo Cluster will probably "rein the errant galaxies in" again.

However, when it comes to galaxy clusters which are much more distant than the Virgo Cluster, the intrinsic motion of the galaxies themselves will not be enough to counteract the expansion of the universe, and all the galaxies in these clusters will have their light redshifted. This means that their net motion is away from us.

Ann

[Chris Peterson]

Is this the same as saying that distant objects will always be red shifted, never blue shifted?

No, objects are red shifted because they are moving away from us (expansion).

There may be a slight Doppler shift- either towards red or blue- because of objects' relative motion with respect to us. But distant galaxies that are redshifted are not showing Doppler shift, and the shift is not because they are moving away from us. We see cosmological redshift because the space through which the photons have traveled to reach us expanded during that duration, stretching out the photon wavelength.

[Chris Peterson]

When it comes to the relatively nearby Virgo Cluster, which is ≈60 million light-years away, the expansion of the universe is enough to make sure that the great majority of galaxies in that cluster have their light redshifted. But because of the large intrinsic motions and velocities within that cluster, a small number of galaxies in the Virgo Cluster are in fact blueshifted. This means that the net motion of the blueshifted galaxies is toward us.

I'd like to pick a nit here, since the subject is a technical one.

All of the galaxies in the Virgo cluster exhibit cosmological redshift because of the expansion of space (which is reduced within the local supercluster because of internal gravitational attraction). Completely separate from this cosmological redshift is Doppler shift, which will be towards longer wavelengths for galaxies with an axial motion away from us, and shorter wavelengths for galaxies moving towards us. This is true for all galaxies and galaxy clusters, of course. What distinguishes the clusters in the local supercluster (and especially the Virgo cluster, because of its high mass) is that the cosmological redshift is on the same order as the Doppler shifts, so in some cases we can see a net shift towards blue. But that shift has to be understood as the sum of two completely different phenomena: cosmological redshift from the expansion of space as the light was traveling towards us, and Doppler shift caused by the motion of the object towards or away from us at the moment the photons were emitted.

For most galaxies, the cosmological redshift is so dominant that Doppler shifts are insignificant, but that isn't the case for the nearby clusters that make up the local supercluster.

[ErnieM]

ErnieM wrote:
Is this the same as saying that distant objects will always be red shifted, never blue shifted?

0=========1=========2=========3==========4==========5==========6
0=========.1=========..2=========...3=========....4=========.....5=========......6

Assuming that objects on space between points 0 to 6 are moving from each other at a constant one period (.) per year per 10 units of distance (=). After one year, a photon travelling from point 6 towards the observer at point 0 will take six more periods(.) than a proton travelling from point 1.
Assuming all photon emitting objects at all 6 points have the same intrinsic movement of X relative to the observer at point 0, this X movement is less and less pronounced the farther the object is from the observer.
Therefore, although space between the observer and the objects are expanding at a constant rate of one (.) per year per 10 units of distance (=), the object at point 6 is red shifted the most.

[petefrederick]

ErnieM wrote:
Is this the same as saying that distant objects will always be red shifted, never blue shifted?

0=========1=========2=========3==========4==========5==========6
0=========.1=========..2=========...3=========....4=========.....5=========......6

Assuming that objects on space between points 0 to 6 are moving from each other at a constant one period (.) per year per 10 units of distance (=). After one year, a photon travelling from point 6 towards the observer at point 0 will take six more periods(.) than a proton travelling from point 1.
Assuming all photon emitting objects at all 6 points have the same intrinsic movement of X relative to the observer at point 0, this X movement is less and less pronounced the farther the object is from the observer.
Therefore, although space between the observer and the objects are expanding at a constant rate of one (.) per year per 10 units of distance (=), the object at point 6 is red shifted the most.

Great explanation Ernie. That cleared up an issue I had. Thanks.

[ErnieM]

Peterfrederick wrote:

Code: Select all

ErnieM wrote:
Is this the same as saying that distant objects will always be red shifted, never blue shifted?

0=========1=========2=========3==========4==========5==========6
0=========.1=========..2=========...3=========....4=========.....5=========......6

Assuming that objects on space between points 0 to 6 are moving from each other at a constant one period (.) per year per 10 units of distance (=). After one year, a photon travelling from point 6 towards the observer at point 0 will take six more periods(.) than a proton travelling from point 1.
Assuming all photon emitting objects at all 6 points have the same intrinsic movement of X relative to the observer at point 0, this X movement is less and less pronounced the farther the object is from the observer. 
Therefore, although space between the observer and the objects are expanding at a constant rate of one (.) per year per 10 units of distance (=), the object at point 6 is red shifted the most.


Great explanation Ernie. That cleared up an issue I had. Thanks.

Thank you. You are so kind.

Above gets more interesting when shown in two and three dimensions of expanding space. Here is an attempt adding another dimension and the two galaxies M and Q at right angle to each other at time = 0.

6 4
= .
= .
= .
= .
5 =
= =
= Q'
= =
= 3
4 .
= .
= .
= =
= =
3 =
= =
= 2
= .
= .
2 =
= =
= =
= =
= 1
1 .
= =
= =
Q =
= =
0 = M = = 1 = = = = 2 = = = = = 3 = = = = 4
0 = = = = . 1 = = = = . . 2 = = = = . . . 3 = M' =

Over the same time period space has expanded at a constant rate but to the observer at M', Q' has move much farther away. In other words, the expansion rate will appear to be accelerating.

Under the assumption that the farther away Q' is, the harder it is to determine its angular momentum, this example puts into question the accuracy of the measurement of "how far back in TIME, or the AGE of galaxy Q' based on the red shift of light.

[Chris Peterson]

Under the assumption that the farther away Q' is, the harder it is to determine its angular momentum, this example puts into question the accuracy of the measurement of "how far back in TIME, or the AGE of galaxy Q' based on the red shift of light.

I don't understand what you are trying to say here. Why are you concerning yourself with angular momentum? That isn't a factor in the redshift-distance relationship.

[ErnieM]

Chris wrote:

I don't understand what you are trying to say here. Why are you concerning yourself with angular momentum? That isn't a factor in the redshift-distance relationship.

I agree, it is not a factor in the redshift-distance relationship. My point is: IF angular momentum of both objects are known, would it not help to confirm the assumption that the objects continue to move at right angle to each other and the estimated redshift-distance of Q' is the hypotenuse? And from this a calculated guess of the rate of expansion?

[Chris Peterson]

I agree, it is not a factor in the redshift-distance relationship. My point is: IF angular momentum of both objects are known, would it not help to confirm the assumption that the objects continue to move at right angle to each other and the estimated redshift-distance of Q' is the hypotenuse? And from this a calculated guess of the rate of expansion?

Do you mean angular momentum or angular velocity?

[ErnieM]

Take you pick. At 7 billions years back both will be very hard to determine.