# Thread: A galaxy a trillion light years away...

1. Could we detect it? (or 999 trillion gazillion light years away if a mere trillion isnt far enough )

How do we know from observation that there is not a galaxy a trillion light years away, or that the universe is not a trillion times larger than what we can observe?

2.

3. The observable universe extends 46 billion light-years from here, in all directions, but we do not think that the observable universe is the whole universe. If inflationary theory is correct, the universe could be many magnitudes larger than our observable part of it, so we might only ever be able to see the tiniest fraction of the whole universe.

So there may indeed be galaxies a trillion light-years away.

Here is something I wrote a few year back which might help you see why.

The observable universe:

Imagine the beginning of time. If light were around, it would take time to reach you, but this is the beginning of time so no light has had time to move yet! Right at the beginning, your observable universe has no size at all! As time moves forward, any light that exists will move at the speed of light. Suddenly you might see a small distance all around you, as light starts coming in from different directions.

After a year, you would be able to see 1 light-year in all directions. After 100 years therefore, your observable universe would be a sphere, 100 light-years in radius. After 13.7 billion years, your observable universe would be 13.7 billion light-years in radius, as you receive light that has been travelling for 13.7 billion years.

Oh, if only it were that simple! The problem comes when considering that the universe is expanding. At the start of things our observable part of the universe was very small and the universe was expanding incredibly fast, much faster than light. Also, light could not move freely until around 370,000 years after the Big-Bang. Before that, photons were frequently interacting with other particles and atoms didn't exist as everything was very hot and mixed up!

But at 370,000 years in, the universe had been expanding and the temperature had cooled enough for atoms to form in a flash of light (the universe finally became transparent and photons first moved freely throughout it). These photons filled the universe at that time, have been passing this way ever since, and we still receive these photons today. They are now stretched into microwaves (by the expansion of the universe) and are known as the Cosmic Microwave Background Radiation (CMBR). All the CMBR was emitted at once, nearly 13.7 billion years ago.

As all this was happening, the universe was expanding. When we worked out how much we thought those CMBR photons had been "stretched" by the expansion, it told us how much bigger the universe is today, than it was when those photons were emitted. We estimate that, when the CMBR was emitted, our observable universe was around 42 million light-years in radius, around 1100 times smaller than it is today!

Hang on though! Didn't I earlier imply that, 370,000 years after the BB, our observable universe would be 370,000 light-years in radius? Well, that radius, based on the time that light takes to travel, is not actually a useful measure of distance at all! In an expanding universe like ours, it is a measure of time elapsed only. When astronomers say the universe is 13.7 billion light-years in radius they are not giving you a distance through space, they are giving you a distance through time, known as the light-travel time. The "actual" distance across an expanding universe, known as the comoving distance, is a different thing entirely (although at distances closer to today, they are essentially the same).

The CMBR photons we receive today have been travelling for nearly 13.7 billion years, but they were emitted at a proper distance of only 42 million light-years away, all that time ago. The reason they have taken so long to reach us is that the universe is expanding, putting more distance in between those CMBR photons and their eventual "targets". Near the beginning of time, if a coordinate point in space was only a few centimetres from another, and those points moved apart with the expansion, then only 370,000 years later those points in space were 42 million light-years apart - that's how fast the universe was expanding, early on!

Then those CMBR photons were emitted throughout the universe and in the case of the ones we detect today, the space they were travelling through was receding from this point in space so fast that, to us, it was as if the photons themselves were receding from us too! The gradual deceleration of the expansion allowed those photons to eventually start making actual progress towards us, from our point of view (they were always travelling away from their original origin point). By the time they found themselves in regions ofspace where an object was receding from us slower than light, they were 5.7 billion light years away from this point in space, and the universe was around 4.5 billion years old! (This is when those photons crossed into our Hubble Sphere as it was at that time).

13.7 billion years after they were originally emitted, 9.1 billion years after they found themselves in space that was receding from us only sub-luminally, we receive those CMBR photons that were only emitted 42 million light-years away. And the real mind-bender is that we think that the original emission point is now over 46 BILLION light-years away. The edge of our observable universe, the most distant point from which we have received CMBR photons, is 46 billion light years away and continues to recede from us. That "edge", known as the surface of last scattering, was receding from this point in space at over 50 times the speed of light when those CMBR photons were emitted, it is still receding at around 3 times the speed of light today and we assume there are galaxies there now, but all we see is the radiation emitted from there, long ago.

The other mind-bender is that the whole universe is probably larger than our observable universe. After a fraction of a second, when our observable universe only had a radius of 10cm, there may well have been the same thing happening 20cm away. When the CMBR was emitted, and our observable universe was only 42 million light-years in radius, there might have been CMBR emitted 80 million light-years away, or much further away than that. Today, when we think our observable universe has a radius of 46 billion light-years and we assume, as galaxies formed in these parts, that there would be galaxies throughout, there could be galaxies whose own observable part of the whole universe is totally separate from ours, galaxies that are 100s of billions of light-years away, outside of our observable part of the universe but still a part of our universe nonetheless.

4. Read Lawrence M Krauss " A Universe from Nothing" In there he explains that in 2 Trillion years the universe will have expanded to a state were no one planetary system will be visible to another.

He makes the point that a Cosmologist starting from scratch in 2 trillion years would be wasting her time. In effect as the sky would be empty, what would prompt a cosmological question?

I fell asleep last night wondering what the nature of the edge of this universe is as it expands into 'nothing' ( Krauss explains his version of nothing as something akin to a vacuum in which matter/energy can pop into and out of existence at will )
Simply put does our universe have a skin?

6. Originally Posted by mikeohare
Simply put does our universe have a skin?
We don't tend to think so. Having a "skin" would imply there is a physical edge, beyond which there is no universe, and this is a very complicated set of affairs to describe. There would be the effect of having all the gravity of the universe on one side of the edge, and none on the other, and the observational evidence for this would propagate inwards.

Cosmologists treat the universe either as being infinite in extent (and full of stuff like it is around here), or if the universe is finite they treat it as if it has no boundary at all, like the surface of the Earth, but stepped up a dimension. If the universe weren't expanding, you might be able to travel in what you think is a straight line, but if you travelled for long enough you might end up back where you started!

7. Im a layman, but if all we can observe were a tiny fraction of a much larger universe, could the background radiation currently attributed to the initial big bang (I think?) be resulting from a huge number of extremely distant galaxies in all directions (like if our observable universe was a grain of sand several inches below the surface on a beach)?

8. Also, if there is a finite distance between superclusters of galaxies, would that not imply a maximum average expansion rate and as a result of that, a maximum range of size? Or is it possible for there to be further levels of clustering that is progressively less bound by gravity, allowing for higher rates of overall expansion and ultimate total size than what we can guess from what we can observe?

In other words, how does the effect of gravity taper off at distances of billions of light years in relation to the effects of expansion?

9. Originally Posted by icewendigo
Im a layman, but if all we can observe were a tiny fraction of a much larger universe, could the background radiation currently attributed to the initial big bang (I think?) be resulting from a huge number of extremely distant galaxies in all directions (like if our observable universe was a grain of sand several inches below the surface on a beach)?
Here's the difficulty with your proposal: The CMB that we observe is a near-perfect fit to a blackbody spectrum, which is the result of thermal equilibrium. In turn, that means that whatever is the source of the CMB had to have interacted long enough to equilibrate. Unless these extremely distant galaxies started off together, interacted to equilibrium, then emitted what we now see as the CMB, your proposal doesn't work. The CMB's shape imposes very strict constraints.

10. Originally Posted by KALSTER
Also, if there is a finite distance between superclusters of galaxies, would that not imply a maximum average expansion rate and as a result of that, a maximum range of size? Or is it possible for there to be further levels of clustering that is progressively less bound by gravity, allowing for higher rates of overall expansion and ultimate total size than what we can guess from what we can observe?

In other words, how does the effect of gravity taper off at distances of billions of light years in relation to the effects of expansion?
Before we go any further, I suggest you read the following link.

Cosmology FAQ: How can the Universe be infinite if it was all concentrated into a point at the Big Bang?

11. Originally Posted by KALSTER
Also, if there is a finite distance between superclusters of galaxies, would that not imply a maximum average expansion rate and as a result of that, a maximum range of size? Or is it possible for there to be further levels of clustering that is progressively less bound by gravity, allowing for higher rates of overall expansion and ultimate total size than what we can guess from what we can observe?
there does seem to be a max size to the structures making up the universe, if that is what you ask.

Observable universe - Wikipedia, the free encyclopedia

there was a paper out on this that i read not that long ago but the filing system my brain uses isn't the best.

12. There are two issues here.

Firstly, the universe is thought to have gone through a period of rapid inflation, within the first fraction of a second of the Big-Bang. Inflation is an extreme form of exponential expansion, where the smallest distances expand at c. Due to this extreme expansion, a lot of universe was put forever out of our view, but we don't know how much.

I have heard estimates that if our pre-inflationary volume was microscopic, that volume was taken and stretched to something like the size of our Solar System, in a fraction of a second. But at the end of inflation, we were left with an observable universe that was only the size of a grapefruit! It is that grapefruit that we measure the expansion of, since. That grapefruit now has a radius of 46 billion light-years!

So, as the grapefruit is to the Solar System, in terms of size, the observable universe is to the pre-inflationary volume. But that is pretty much a minimum bound!

It was during inflation that the universe was "smoothed out" by that rapid "stretching", which explains the smoothness of the CMBR. But quantum fluctuations were also magnified, which allow the slight differences that allowed large scale structure to form later.

Secondly, and even more fundamentally, we don't know how much of the whole universe our pre-inflationary volume took up. The whole universe could have been any size, even infinite. Before inflation. But our pre-inflationary volume was microscopic. And post inflation, that volume was at least the size of the Solar System, and our observable universe only formed a grapefruit sized part of it.

13. Speedfreak. Do you have any surplus brain cells? I still have trouble reading the Beano

14. PS What do you mean by the whole universe? The only way I can cope with the existence of 'our' universe is to accept there are an infinite number of universes. Does your whole encompass all?

15. By "whole" universe, I am simply referring to the volume that took part in the Big-Bang, of which our observable universe is just a small part (we don't know how small a part).

If you are referring to "multiverse" concepts, then our "whole" universe is just one of many universes, and our observable universe is only a small part of one of those universes.

16. Originally Posted by SpeedFreek
The observable universe extends 46 billion light-years from here, in all directions, but we do not think that the observable universe is the whole universe. If inflationary theory is correct, the universe could be many magnitudes larger than our observable part of it, so we might only ever be able to see the tiniest fraction of the whole universe.

So there may indeed be galaxies a trillion light-years away.

Here is something I wrote a few year back which might help you see why.

The observable universe:

Imagine the beginning of time. If light were around, it would take time to reach you, but this is the beginning of time so no light has had time to move yet! Right at the beginning, your observable universe has no size at all! As time moves forward, any light that exists will move at the speed of light. Suddenly you might see a small distance all around you, as light starts coming in from different directions.

After a year, you would be able to see 1 light-year in all directions. After 100 years therefore, your observable universe would be a sphere, 100 light-years in radius. After 13.7 billion years, your observable universe would be 13.7 billion light-years in radius, as you receive light that has been travelling for 13.7 billion years.

Oh, if only it were that simple! The problem comes when considering that the universe is expanding. At the start of things our observable part of the universe was very small and the universe was expanding incredibly fast, much faster than light. Also, light could not move freely until around 370,000 years after the Big-Bang. Before that, photons were frequently interacting with other particles and atoms didn't exist as everything was very hot and mixed up!

But at 370,000 years in, the universe had been expanding and the temperature had cooled enough for atoms to form in a flash of light (the universe finally became transparent and photons first moved freely throughout it). These photons filled the universe at that time, have been passing this way ever since, and we still receive these photons today. They are now stretched into microwaves (by the expansion of the universe) and are known as the Cosmic Microwave Background Radiation (CMBR). All the CMBR was emitted at once, nearly 13.7 billion years ago.

As all this was happening, the universe was expanding. When we worked out how much we thought those CMBR photons had been "stretched" by the expansion, it told us how much bigger the universe is today, than it was when those photons were emitted. We estimate that, when the CMBR was emitted, our observable universe was around 42 million light-years in radius, around 1100 times smaller than it is today!

Hang on though! Didn't I earlier imply that, 370,000 years after the BB, our observable universe would be 370,000 light-years in radius? Well, that radius, based on the time that light takes to travel, is not actually a useful measure of distance at all! In an expanding universe like ours, it is a measure of time elapsed only. When astronomers say the universe is 13.7 billion light-years in radius they are not giving you a distance through space, they are giving you a distance through time, known as the light-travel time. The "actual" distance across an expanding universe, known as the comoving distance, is a different thing entirely (although at distances closer to today, they are essentially the same).

The CMBR photons we receive today have been travelling for nearly 13.7 billion years, but they were emitted at a proper distance of only 42 million light-years away, all that time ago. The reason they have taken so long to reach us is that the universe is expanding, putting more distance in between those CMBR photons and their eventual "targets". Near the beginning of time, if a coordinate point in space was only a few centimetres from another, and those points moved apart with the expansion, then only 370,000 years later those points in space were 42 million light-years apart - that's how fast the universe was expanding, early on!

Then those CMBR photons were emitted throughout the universe and in the case of the ones we detect today, the space they were travelling through was receding from this point in space so fast that, to us, it was as if the photons themselves were receding from us too! The gradual deceleration of the expansion allowed those photons to eventually start making actual progress towards us, from our point of view (they were always travelling away from their original origin point). By the time they found themselves in regions ofspace where an object was receding from us slower than light, they were 5.7 billion light years away from this point in space, and the universe was around 4.5 billion years old! (This is when those photons crossed into our Hubble Sphere as it was at that time).

13.7 billion years after they were originally emitted, 9.1 billion years after they found themselves in space that was receding from us only sub-luminally, we receive those CMBR photons that were only emitted 42 million light-years away. And the real mind-bender is that we think that the original emission point is now over 46 BILLION light-years away. The edge of our observable universe, the most distant point from which we have received CMBR photons, is 46 billion light years away and continues to recede from us. That "edge", known as the surface of last scattering, was receding from this point in space at over 50 times the speed of light when those CMBR photons were emitted, it is still receding at around 3 times the speed of light today and we assume there are galaxies there now, but all we see is the radiation emitted from there, long ago.

The other mind-bender is that the whole universe is probably larger than our observable universe. After a fraction of a second, when our observable universe only had a radius of 10cm, there may well have been the same thing happening 20cm away. When the CMBR was emitted, and our observable universe was only 42 million light-years in radius, there might have been CMBR emitted 80 million light-years away, or much further away than that. Today, when we think our observable universe has a radius of 46 billion light-years and we assume, as galaxies formed in these parts, that there would be galaxies throughout, there could be galaxies whose own observable part of the whole universe is totally separate from ours, galaxies that are 100s of billions of light-years away, outside of our observable part of the universe but still a part of our universe nonetheless.
That was an interesting read, and no doubt will have to read it a few more times to fully comprehend. One thing I would ask is are you saying the expansion of the universe is faster than the speed of light?

Anthony

17. But I dont understand why Galaxies arent stretching as well? And Me, am I a gargantuan on a gargantuan planet now but was a Lilliputian of a Lilliputian planet? Is the space between atoms increasing and if not why not? If the earth is getting bigger, is it pushing me as it expands?

Is anti-matter travelling backwards in times and crunching in a gargantuan cosmic black hole, and is the expansion of the universe the opposite of a black hole creation? (if space can stretch on a cosmic scale, can space shrink in a black hole?)

Wouldnt it be the same as an expansion, if a hypothetical center of the universe was shrinking? So that being halfway between the middle and the edge, external galaxies look no bigger because the space as its light is coming towards you is shrinking down to your size while galaxies closer to the center emit light that is expanding?

18. Originally Posted by AnthonyChan
One thing I would ask is are you saying the expansion of the universe is faster than the speed of light?

Anthony
Yes.

See [astro-ph/0310808] Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe for a formal explanation of the standard cosmology.

We use standard general relativity to illustrate and clarify several common misconceptions about the expansion of the Universe. To show the abundance of these misconceptions we cite numerous misleading, or easily misinterpreted, statements in the literature. In the context of the new standard Lambda-CDM cosmology we point out confusions regarding the particle horizon, the event horizon, the ``observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests. Attempts to restrict recession velocities to less than the speed of light require a special relativistic interpretation of cosmological redshifts. We analyze apparent magnitudes of supernovae and observationally rule out the special relativistic Doppler interpretation of cosmological redshifts at a confidence level of 23 sigma.
Or you could have a look at the pdf I posted below, for a more user-friendly description.

19. Originally Posted by icewendigo
But I dont understand why Galaxies arent stretching as well?
In addition to the paper I referenced in the above post, the authors wrote a popular science article based on that paper, which was published in Scientific American magazine. The pdf file below contains that article and it addresses this issue.

See the section titled "is Brooklyn expanding?"
http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf
(note: the first page of this pdf file is blank - don't be confused into thinking it has failed to load!)

20. But I dont understand why Galaxies arent stretching as well? And Me, am I a gargantuan on a gargantuan planet now but was a Lilliputian of a Lilliputian planet? Is the space between atoms increasing and if not why not? If the earth is getting bigger, is it pushing me as it expands?
on the small scale electrostatic forces that keep atoms together and gravity on the large scale that keep solar systems and galaxies together are stronger than the forces making the expansion happen. it is only in the intergalatic spaces where gravity effects are weak enough that the expansion forces dominate.

21. @SpeedFreak: Fascinating, quite thought provoking to those of us in the laity.

Is there any clue as to the vector from which the universe began (magnitude/direction)?

If so, can we calculate the approximate position of our solar system relative to to that point in relation to time?

Just wondering, I have no astronomical background whatsoever.

22. Triola, there is no location where the Universe began that we can point to and say "there". there is no centre. when the BB happened everywhere came into existence at the same time. the BB did not expand in pre-existing space from a point.

23. Are forces that repel and attract atoms both alternating between repulsion and attraction and get weaker but project over longer distances?
(the greater the effective range of the force the weaker it is? The shorter the range of effect the stronger it is?)

24. (the greater the effective range of the force the weaker it is? The shorter the range of effect the stronger it is?)
electromagnetism and gravity get weaker the further apart the objects are, inverse square law. the strong force gets stronger the further apart the quarks are.

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