# Why distant star's light can reach us?

#### Saint

Valued Senior Member
1. Theory says the universe is expanding faster than light's speed.
2. If (1) is true, why we can see stars ten of billions years away? (universe life 13.8 billion years)
I assume the star's light is not able to reach us because the space between them and us are getting wider faster than the speed of light..
3. How do we know the universe is 13.8 billion years old? How to measure it?

You have to be very carefull when you talk about speed of distant objects.
This kind of speed is not real but a calculation with no real physical meaning.
Only local properties have real meaning.
This speed, if you do "the calculation", can be bigger than the speed of light.
You can have distant places moving faster than the light speed from each other (if you do the calculation), but the light can travel between these places and finaly arrive at the distant place.
In this case, light will only take more time to arrive, but it will.

The only real speed is the one occuring localy at the place where it is measured relativ to the LOCAL environment.
If the local expansion exceed the speed of light, yes, in this case the light can not advance any more.

1. Theory says the universe is expanding faster than light's speed.
Correct.
2. If (1) is true, why we can see stars ten of billions years away? (universe life 13.8 billion years)
Actually we are seeing light from those stars that is ten billions years OLD. The stars were much closer when the light was emitted and the stars are much farther away now. The light emitted from those stars 'now' will never reach us because their recession velocity is higher than c.
3. How do we know the universe is 13.8 billion years old? How to measure it?
Seems like you have asked this before, so instead of explaining it again try reading this: https://wmap.gsfc.nasa.gov/universe/uni_age.html#:~:text=Astronomers estimate the age of,the holes in a wall.

1. Theory says the universe is expanding faster than light's speed.
Theory confirmed by many observations.
2. If (1) is true, why we can see stars ten of billions years away? (universe life 13.8 billion years)
Because, as origin said, those stars weren't tens of billions of years away when light started travelling from them to us. The universe is expanding.
I assume the star's light is not able to reach us because the space between them and us are getting wider faster than the speed of light.
Not everywhere. The rule is that the further away something is from us, the faster we see it receding. There are lots of things close enough to us that they aren't receding at greater than the speed of light.
3. How do we know the universe is 13.8 billion years old? How to measure it?
1. Measure the distances to a whole lot of different objects we can see (galaxies will do nicely).
2. Measure the speed at which each of those objects is receding from us.
3. Work out how long ago each of those objects was right here, where we are.
4. Find answer: 13.8 billion years.
5. Done.

You have to be very carefull when you talk about speed of distant objects.
This kind of speed is not real but a calculation with no real physical meaning.
This is incorrect. A speed is a speed is a speed. One speed is a "real" as any other.
Only local properties have real meaning.
No.
You can have distant places moving faster than the light speed from each other (if you do the calculation), but the light can travel between these places and finaly arrive at the distant place.
No, it can't.
The only real speed is the one occuring localy at the place where it is measured relativ to the LOCAL environment.
No.
If the local expansion exceed the speed of light, yes, in this case the light can not advance any more.
What does "local" mean, in this context?

Look, Dicart: if you don't know the answers to science questions, don't pretend like you do and post in our Science subforums. Okay? Because wrong answers can mislead other readers, and we don't want that.

This is incorrect. A speed is a speed is a speed. One speed is a "real" as any other.

There is instant speed, average speed, velocity, celerity, recession speed, etc
You never learned the basic ?
Only local properties are real, other considerations are mathematical points of view.
Thats why you believe in fairytales.
And thats why C is localy limited and other points of view permit higher "speed".

No, it can't.

Perhaps, or perhaps not.
It depend on the final fate of the universe.
But if you dont take (full reality) this in account, perhaps it is because you have a narrow mind ?

Sure it is.

What does "local" mean, in this context?
What does "local" mean, in this context?

It means that if there is a local expansion (like in a black hole) that can counteract the local expansion at quantum scale, the one ( expansion) that permit to light to advance, light do not advance any more.

Look, Dicart: if you don't know the answers to science questions, don't pretend like you do and post in our Science subforums. Okay? Because wrong answers can mislead other readers, and we don't want that.

But you can freely give the standard good (false) answer, we dont even need your advise or go to the sciforums, all is already writen everywhere. And in some years we will not even need humans to explain these faitytales, IA is sufficient to act like parrots.

There is instant speed, average speed, velocity, celerity, recession speed, etc
You never learned the basic ?
Velocity is not the same as speed. Velocity is a vector and speed is a scalar.
You never learned the basic ?

Stop trolling and spend your time on something more productive, like learning basic physics. Bye and I wish I didn't take you off of ignore, won't happen again...

Correct.

Actually we are seeing light from those stars that is ten billions years OLD. The stars were much closer when the light was emitted and the stars are much farther away now. The light emitted from those stars 'now' will never reach us because their recession velocity is higher than c.

Seems like you have asked this before, so instead of explaining it again try reading this: https://wmap.gsfc.nasa.gov/universe/uni_age.html#:~:text=Astronomers estimate the age of,the holes in a wall.

Because, as origin said, those stars weren't tens of billions of years away when light started travelling from them to us. The universe is expanding.
If you say star-XYZ is 10 billion light-year away, its light can reach us because it was nearer to us when it emitted the light we see now, assuming 3 billion light-year.
But, why astronomer tells us it is 10 billion light-year away? It should be <10 billion light-year because what we see is the light of earlier age.

When an old star was discovered, astronomer tell us it is very far away, example 12 billion light-year away, but as you said, light from 12 billion light-year away may not have chance to reach us because the universe is expanding faster than c.
Does astronomer do extrapolation calculation to estimate its current position?

Same as a fossil of an ancient creature was found, the scientist tell you that it is 50 millions year ago?
How did they know for so sure?
Simply plug from the air?
What is the error of measurement?

Same as star's distance, what is the error of measurement? What tolerance?

Same as a fossil of an ancient creature was found, the scientist tell you that it is 50 millions year ago?
How did they know for so sure?
Simply plug from the air?
What is the error of measurement?

Same as star's distance, what is the error of measurement? What tolerance?
Standard candles: https://astronomy.swin.edu.au/cosmos/S/Standard+Candle

Margin of error will depend on the specific measurement.

Do we benchmark star's distance with known star (which we can measure accurately) ?

If you say star-XYZ is 10 billion light-year away, its light can reach us because it was nearer to us when it emitted the light we see now, assuming 3 billion light-year.
But, why astronomer tells us it is 10 billion light-year away? It should be <10 billion light-year because what we see is the light of earlier age.
The star might have been 3 billion light years away from us when the light we see was emitted by it. But that was 10 billion years ago (say). While the light was travelling from where it was emitted towards the Earth, the star that emitted it continued moving away from the Earth, and the distance between the point of emission and the Earth also expanded as the light travelled. So the 10 billion light-year distance is the distance to where the star is now (as we receive the light), not the distance to where the star was when the light was emitted.
Does astronomer do extrapolation calculation to estimate its current position?
Well, yes and no. For example, we can see emission lines in the spectrum of a star. From the identified emission lines, which are red-shifted due to the expansion of the universe between the time the light was emitted and when we received it, we can work out how fast the star is moving away from us (now). Since there is a known relationship between the speed of recession of distant objects and their distance from us, we can calculate the distance to the star (now).
Do we benchmark star's distance with known star (which we can measure accurately) ?
Yes. There is a long chain of "standard candles" that are used to establish the distances to objects seen in space.

"C " speed of light is constant.
While traveling in space, the space-time fabric is expanding too,
there might be obstacles between the star and the earth, objects that can absorb the light or reflect/deflect the light, maybe it pass by a blackhole and being curved away from original direction,
in this case, how can we be sure of its distance and direction?

"C " speed of light is constant.
While traveling in space, the space-time fabric is expanding too,
there might be obstacles between the star and the earth, objects that can absorb the light or reflect/deflect the light, maybe it pass by a blackhole and being curved away from original direction,
in this case, how can we be sure of its distance and direction?
If it is absorbed, we won't see it at all.
The gravitational lensing (deflection) of light requires huge amounts of mass. We're talking entire galaxies, that kind of thing. We do see some light that has been deflected in that way. Obviously, again, light that is deflected away from our line of sight is never seen by us. We can often tell what it is that has deflected the light (e.g. it's hard to hide a galaxy). We can also work backwards to work out where the light probably came from if it was deflected by a known object.

"C " speed of light is constant.
While traveling in space, the space-time fabric is expanding too,
there might be obstacles between the star and the earth, objects that can absorb the light or reflect/deflect the light, maybe it pass by a blackhole and being curved away from original direction,
in this case, how can we be sure of its distance and direction?
Objects are not static in space. Any alignment such as with a black hole would be fleeting when it comes to individual stars. That being said, the fact that distance measurements cannot be 100% accurate is already accounted for. For example, if you look up the distance to one of our neighboring stars, Sirius, you'll see its distance listed as being 8.709 ly plus or minus 0.005 ly. Polaris, the north star is listed as being 323-422 ly away. The Messier 87 galaxy is 58.5 million ly, plus or minus 1.6 ly, etc.

1) I read that the entire universe is not a sphere, but similar to the thin shell on the surface of a sphere, right?
2) The solar system is in the Milky Way galaxy, how can we take picture of the whole galaxy? Those pictures of milky way, are they fake? If you are inside a car, how can you take picture of the whole car? You must come out from the car to take picture of it.

1) I read that the entire universe is not a sphere, but similar to the thin shell on the surface of a sphere, right?
2) The solar system is in the Milky Way galaxy, how can we take picture of the whole galaxy? Those pictures of milky way, are they fake? If you are inside a car, how can you take picture of the whole car? You must come out from the car to take picture of it.

I quote below the part relevant to your question:-

The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching the night sky.[53] In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way Galaxy, the term "Milky Way" is limited to this band of light.[54][55] The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Brighter regions around the band appear as soft visual patches known as star clouds. The most conspicuous of these is the Large Sagittarius Star Cloud, a portion of the central bulge of the galaxy.[56] Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. The area of sky that the Milky Way obscures is called the Zone of Avoidance.[57]

So what you see is a part of the galaxy, not the whole thing. Obviously.

1) I read that the entire universe is not a sphere, but similar to the thin shell on the surface of a sphere, right?
I think you're probably confusing an analogy that is sometimes used to describe how the universe expands with the actual shape of the universe.

The universe seems to be infinite in size, as far as we can tell. Therefore, it doesn't have any particular shape. We can't see the universe from the outside.

Spacetime as a whole is very close to being "flat" (in the technical sense of spacetime curvature). This means the universe is "open", in that if you keep travelling in one direction you'll never come back to where you started. If spacetime was spherical, then in principle you could effectively "complete the loop" and find yourself back where you started after travelling in the same direction (similar to how you can travel around the equator of the Earth in one direction and find yourself back where you started).
2) The solar system is in the Milky Way galaxy, how can we take picture of the whole galaxy?
We can't. At least, not all at once. Our solar system is about 2/3 of the way out from the centre of our galaxy, so we see many more stars when we look towards the centre (which is in the direction of the Sagittarius constellation) compared to when we look outwards towards the edge.
Those pictures of milky way, are they fake?
No. You're seeing a view of the galaxy "side on", looking from where we are towards the centre.
If you are inside a car, how can you take picture of the whole car? You must come out from the car to take picture of it.
Right. Same with our galaxy.

When scientists do computer modeling or simulation of the Big Bang, it starts with an explosion of a singularity point, won't that modeling will end up with a Sphere?