# What is the relationship between light and space?

1. Apr 2, 2013

### Seminole Boy

What is light doing to make us believe that space is boundless but also finite (if I have Einstein's thinking right)?

There's something tricky going on here.

Are there points (coordinates) in space that are not reached by light?

2. Apr 2, 2013

### Staff: Mentor

There is no reason to believe that space is finite. It might be, but it could be infinite as well.
There are good arguments to believe that space is boundless - space looks the same in all directions (in every part we can observe), and a bound would be a special region in space with strange effects.

Every part can be reached by light emitted nearby. There are parts of space we can never reach with light sent towards them, as they are too far away and space is expanding too quickly in between.

3. Apr 2, 2013

### Seminole Boy

MFB:

So, yes, I see where you said that these parts of space are already far away, but your "space is expanding too quickly in between" seems to suggest that space is expanding faster than the speed of light. Please do explain further.

4. Apr 2, 2013

### Staff: Mentor

The distance between us and objects far away is increasing faster than the speed of light, right*.
This is not a problem - relativity limits the speed of objects in space, not the expansion of space itself.

*strictly speaking, there is not just one unique way to define those distances - you can choose different coordinate systems, where this is not true.

5. Apr 3, 2013

### Naty1

So far as we know, relic radiation, that is electromagnetic radiation [light]from the big bang is everywhere in our universe. It's wavelength is getting longer as distances expand and so it is getting weaker....longer wavelength means less energy. It started out close to 3,000 K and is now under 3 degrees K.

So in the far distant future the universe will be cold, dark, dead....nothing will be happening if the expansion we believe exists continues as we believe it will...forever....

6. Apr 3, 2013

### Naty1

In the sense that light from beyond about 16 billion light years distance will never reach us, yes.

7. Apr 4, 2013

### Staff: Mentor

To elaborate a little bit on what has been said already:

Expansion is measured by a RATE, not a speed or velocity. What this means is that if you take a volume of space and choose any number of points within it, every point will get further away from every other point over time. For example, we could say that it takes X amount of time for any points within a volume of space to get twice as far away from each other. However we usually choose a point away from us and, knowing this expansion rate, say that any object at that point is receding from us at a given velocity.

For the universe, expansion results in an increase in the recession velocity of objects by about 67 km/s for every megaparsec in distance away from an observer. IE, an object at one megaparsec, which is 1 million parsecs, or about 3.26 million light-years, will be receding from us at about 67 km/s. Increase the distance to 2 megaparsecs and the recession velocity increases to 134 km/s. At 10 megaparsecs it's 670 km/s.

Now, light travels at about 300,000 km/s. That means that any object that at a distance greater than about 4500 megaparsecs, or about 14 billion light-years, is receding from us at a velocity GREATER than c. This does NOT violate the rule that nothing can travel faster than light. Why? Because that rule only applies to Special Relativity, not to General Relativity. GR allows the geometry of spacetime to cause objects to move away at ANY velocity. This is commonly explained as "Nothing can move THROUGH space faster than c, but expansion causes space itself to carry objects away".

8. Apr 4, 2013

### Naty1

That's a difficult concept to wrap your mind around. The following complements Drakkith's post and is not intended to refute his explanation.

In the old thread #162727 in these forums,

There is a wonderful post by pervect, #90 about different models providing superluminal or sub luminal speeds, and clarifies for ‘non experts’ a difficulty of interpreting ‘distance’ in cosmology: [and hence velocity, energy,etc]

Further, it turns out you can visualize 'expansion' via the SCALE FACTOR a[t] which results from the FLRW cosmological model, selected parameters, and the Einstein Field Equations.
Just visualize the scale factor a[t] as a coordinate distance between a pair of space time points.
For example, the scale factor for a matter-dominated universe, {an approximate expression, not exactly Lambda CDM} goes as

a(t) = (t/to)2/3 so as time t grows arbitrarily large, so does a[t]. This means separation distances grow faster than lightspeed....stuff becomes 'superluminal'....

[This is NOT a typical d = v[t] type measure so common in flat space.]

Leonard Susskind derives such expression in 'Youtube Susskind Cosmology' lecture #3.....but it is awfully slow and that one lecture about 2 hours....

Last edited: Apr 4, 2013