the big bang

[251billyg]

My question is-if the universe started as an infinitely dense singularity-and bang-it goes in all directions-and we now can see light from a galaxy 13 billion ly away-how could it get that far unless it [the distant galaxy] was traveling at near light speed for 13 billion years?

[rstevenson]

My question is-if the universe started as an infinitely dense singularity-and bang-it goes in all directions-and we now can see light from a galaxy 13 billion ly away-how could it get that far unless it [the distant galaxy] was traveling at near light speed for 13 billion years?

The galaxy wasn't travelling in the sense you mean. The space in which that galaxy (and everything else including us) exists has been expanding.

Rob

[Ann]

Let's try to put it this way. Space is allowed to expand at any velocity, and it can carry everything along with it at its own speed when it expands.

But anything made of matter inside the universe - stars, planets, moons, people, whatever, including galaxies - is not able to travel faster than light, or even as fast as light, in relation to the local universe.

Light may - and does - travel at the speed of light in relation to the local universe, but it can travel no faster.

Why does the universe work this way? Ask someone who knows the answer.

Ann

[Chris Peterson]

My question is-if the universe started as an infinitely dense singularity-and bang-it goes in all directions-and we now can see light from a galaxy 13 billion ly away-how could it get that far unless it [the distant galaxy] was traveling at near light speed for 13 billion years?

There is no restriction on how fast one bit of matter can travel with respect to another. As the Universe expands, it carries the matter it contains apart. All of the early galaxies we see in images are currently much farther away from us than 13 billion light years, and are moving away from us at much faster than c.

This sort of "moving" is quite different from what we usually mean, where one object experiences a force, causing acceleration and moving it away from another. That sort of conventional movement can never result in a relative speed between the objects greater than c. In a classical physics sense, we could even say that nothing in the Universe is moving apart at all, it's the metric of the Universe itself that is changing... imagine that the ruler you use to measure distance is getting smaller all the time. You'd say that distances are increasing, even though nothing is actually changing.

[saturno2]

I had prometed not to disturb the corrent theory of the
Big Bang, but I found something very interesting.
Roger Penrose says that the Big Bang is a cyclical
phenomenon.
There have been several Big Bang in the History
of the Universe.
Each comprising a cycle call < eon >
There is not a only Big Bang ( perhaps )

[Chris Peterson]

I had prometed not to disturb the corrent theory of the
Big Bang, but I found something very interesting.
Roger Penrose says that the Big Bang is a cyclical
phenomenon.
There have been several Big Bang in the History
of the Universe.
Each comprising a cycle call < eon >
There is not a only Big Bang

It's just a hypothesis, not even rising to the level of a theory. Idle speculation to stimulate thought.

[saturno2]

I had prometed not to disturb the corrent theory of the
Big Bang, but I found something very interesting.
Roger Penrose says that the Big Bang is a cyclical
phenomenon.
There have been several Big Bang in the History
of the Universe.
Each comprising a cycle call < eon >
There is not a only Big Bang

It's just a hypothesis, not even rising to the level of a theory. Idle speculation to stimulate thought.

In any case it is an important hypothesis to breaks
a little the actual untouchable Big Bang theory.

[rstevenson]

In any case it is an important hypothesis to breaks
a little the actual untouchable Big Bang theory.

The Big Bang Theory is not untouchable; it's just very well supported. Which is true of all scientific hypotheses that have risen to the level of theory.

Rob

[AndyMac]

My question is-if the universe started as an infinitely dense singularity-and bang-it goes in all directions-and we now can see light from a galaxy 13 billion ly away-how could it get that far unless it [the distant galaxy] was traveling at near light speed for 13 billion years?

The galaxy wasn't travelling in the sense you mean. The space in which that galaxy (and everything else including us) exists has been expanding.

Rob

How can you tell that space is expanding unless you have a non-expanding frame of reference?

[Chris Peterson]

How can you tell that space is expanding unless you have a non-expanding frame of reference?

You can tell because the wavelength of radiation is increasing with distance. It is getting stretched out by expansion.

[Beyond]

So the farther a wave length travels, the more stretched out it gets?

[Chris Peterson]

So the farther a wave length travels, the more stretched out it gets?

Basically. And since space and time are linked by the speed of light, you could also say that the longer a photon has been traveling, the longer its wavelength.

[geckzilla]

It seems like it would be a pain to separate redshift due to distances and redshift due to Doppler effects.

[Chris Peterson]

It seems like it would be a pain to separate redshift due to distances and redshift due to Doppler effects.

It is. And depending on the choice of reference frames, the two may be seen as the same. In reality, we understand that redshift (or blueshift) at small scales is dominated by Doppler (and analyzed using the equation for Doppler shift), while redshift at very long scales is dominated by cosmological redshift (and analyzed using the equation for that). But there is a boundary zone between the two were both are significant.

Of course, there's also gravitational redshift, but it's usually not a factor except in very special cases.