Celestia Forums

Greetings! I know I'll probably get laughed at for asking this question, but there goes... Let's say I were to look into a telescope, and I were to see a galaxy say, 500,000 light years away. The image of the galaxy that I would see would be the galaxy how it was 500,000 years ago, correct? In other words, I wouldn't be seeing what the galaxy might look like today? Thanks.

Yes, you'd see it as it was 500,000 years ago. Nothing travels faster than light so the light you see is carrying the latest available information about the galaxy you can get.

Brendan

Well maybe it's seems simple, but (in my point of view) it's definitely not

Let's take it in an simple and intuitive (but wrong) way :

Let's imagine that after the big bang, the Universe is always expanding at light speed, then imagine we are at the border of the expanding Universe. Then we always have this equation:
our distance to this center (in light years) = time for the first big bang light to come to us (in years).
So we can look at anytime at the center of the Universe and see the first bigbang light.

The problem is:
- Universe is not expanding at light speed, right ?
- Universe does not have a center, right ? Then could the first big bang light be observed anywhere or nowhere ? Really, how far/back in time could we see if we were not limited by the instruments ?

Anonymous wrote:Well maybe it's seems simple, but (in my point of view) it's definitely not

Let's take it in an simple and intuitive (but wrong) way :

Let's imagine that after the big bang, the Universe is always expanding at light speed, then imagine we are at the border of the expanding Universe. Then we always have this equation:
our distance to this center (in light years) = time for the first big bang light to come to us (in years).
So we can look at anytime at the center of the Universe and see the first bigbang light.

The problem is:
- Universe is not expanding at light speed, right ?
- Universe does not have a center, right ? Then could the first big bang light be observed anywhere or nowhere ? Really, how far/back in time could we see if we were not limited by the instruments ?

while I usually do not tend to discuss with nameless people, cyber robots or shell scripts , here are nevertheless a couple of issues to consider for your question:

1) You may have overlooked, that one of the most spectacular recent experiments in this area was the quantitative recording of the cosmic microwave background radiation ( CMB) from BOOMERANG, WMAP,... This is a /direct/ testimony of the BigBang or very close to it: the universe was then merely 300000 years old! It's size was only about 1 degree in the sky as seen from here. Of course one cannot see any light earlier than that, since the hot and dense plasma of the early universe was NOT transparent to light. The CMB signal is just from the time threshold where the decoupling of photons from the plasma took place...

2) Of course you MUST apply general relativity to your considerations, otherwise you sure get nonsense.

Based on the assumption that the universe is homogeneous and isotropic on large scales, the framework for modern cosmology rests on Einstein's general theory of relativity, leading to the successful hot Big Bang or Friedmann- Robertson-Walker cosmological model. The dynamics of the expanding universe are described by the Friedmann equation, relating the expansion rate to the density and curvature of the universe.

Today's expansion rate of the Universe is given by the Hubble constant, that can be directly measured with various methods.

The expansion rate would only equal the speed of light in /empty space/ with the gravitational force neglected! While the speed of light is always the same, the high initial density of mass induced a strong /curvature/ of space-time along with many other important effects.

You must also take into account that gravity was a strong force at the very short distances corresponding to right after the big bang. Only at the huge distances involved in today's cosmology, gravity acts extremely weakly.

Bye Fridger

1. The timespace ITSELF is expanding. There is nothing that expands INTO a timespace.
2. The universe therefore has no center or the whole universe was the center once.
3. So everywhere you look at, you look at the former center.
4. The first big bang light CAN be observed.
5. You can look anywhere, because the microwave backround ratiation IS the first big bang light.
6. The existence of the microwave background is one if the pillars the big bang theory is based on.

An (essentially wrong but simple and easy to unterstand) example:
Imagine the universe (the spacetime) is the hull of a balloon that is slowly but constantly beeing inflated. The hull will grow continuously but have no center. The hull will be endless but not infinite. Watched from 'outside' you can see something like a center, but this center is not part of the hull - the universe/spacetime. You can imagine it as something extradimensional if you like, but nothing you can proof or measure.

maxim

OK, seems I was a bit slower than Fridger

maxim

I like your answers, maxim, they are very clear and interesting. You're a teacher, right ?

Thank you too toofry