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"Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion

"Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us." This is false, because it ignores the metric expansion of spacetime. We can see radiation from objects that are now ~46.5 billion light-years from Earth. This isn't picking nits. The explanation given here makes it sound like the Universe is a Euclidean volume that got instantaneously filled with stars, at roughly their present density, and that this density is low enough that light from distant stars hasn't reached us yet. If this were true, it wouldn't only explain why the sky is dark now, but predict that it has always been dark. We know this is false: for the first 380,000 years after the Big Bang, the Universe was filled with hydrogen plasma and completely opaque to radiation, which we can still see in the form of CMBR. As spacetime expands, energy density decreases, space becomes less light-emitting and more transparent, and light already in transit becomes red-shifted. Our field of vision is still technically flooded with light, as it always has been, but by now the red-shift is so extreme that the light is in the radio spectrum.

kyberias 11y ago
> As spacetime expands, energy density decreases, space becomes less light-emitting and more transparent, and light already in transit becomes red-shifted. Our field of vision is still technically flooded with light, as it always has been, but by now the red-shift is so extreme that the light is in the radio spectrum. You talk about the radiation generated before 1M years in the universe. This article talked about the light from the existing stars in the universe.
- goodside 11y ago
  The red shift applies to distant stars too. The important point is that even in the extreme scenario where all stars last forever and you wait an infinite amount of time for light to reach you, you will still never see a fully illuminated sky because the stars are receding much faster than the light propagates due to metric expansion. Saying the sky is dark because 1) the Universe is young or 2) stars are short-lived is just false. The Universe gets darker and darker as it expands, despite there being "more time for light to reach us".
- notalaser 11y ago
  Yes, it's still incorrect! The complete quote is: > Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us. There may be an infinite number of stars beyond that cosmic horizon but we can’t see them because their light has not yet arrived. And the observable part of the universe contains too few stars to fill up the sky with light. (Emphasis mine because that's where the error is: 13 billion year-old light has traveled over a distance that is now much higher than than 13 billion light-years) Potential ambiguity (in cases such as this one) is one of the reasons why using light-years for distances may not be a good idea. (By the way, there are real astronomers arguing about this, too, e.g. http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html . So it's not just this nerd ranting on HN). GRB 090423 (admittedly, not a star -- it's a gamma ray burst) happened approximately 13 billion years ago. This means that light traveled for 13 billion years to reach us -- but, by the time it got to us, GRB 090423 was no longer 13 billion light-years from us, because all that space got bigger! GRB 090423 "was" about 9.2 Gpc away from us by the time that light reached us, i.e. about 30 billion light-years. If the light that hits us is 13 billion years old, that object (assuming it still exists) is a lot farther than 13 billion light-years from us now.
  - vectorjohn 11y ago
    Sure. But that really is just pedantics. It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. You are essentially just giving those words the least charitable interpretation and then picking on that. It isn't wrong, its just not perfectly unambiguous.
    - notalaser 11y ago
      > It isn't wrong, its just not perfectly unambiguous. Yes, it's perfectly wrong, because those objects are not 13Gly away from us. A light-year really is defined as 300,000 km multiplied by whatever number of seconds there are in a year. The object is not at 13e9 x 300,000 x <a lot of seconds> kilometers from us if it took 13 billion years for the light to reach us. > It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. If I wanted to give words the least charitable interpretation, I could point out that their motion has nothing to do with this -- they would still be farther than 13 Gly away from us even if they would not be moving away relative to the Earth :-).
joering2 11y ago
No, its correct. I lost a youtube video that explains Universe. Basically a universe observer is like an ant sitting on a big balloon that is constantly being pump with air. So while the balloon age might be 12 billion years old, the speed the air is pumped in it makes it grow and therefore objects "placed" far further than originally; hence 46B for an observer.
jerf 11y ago
At the risk of going more-pedantic-than-thou, this entire thread of nitpicks (and indeed, most of the posts I see right now to this story in general) is(/are) mostly irrelevant. The argument is not particularly sensitive to the exact way in which the universe is not steady state, including redshifting all the light until we're not staring at the sun in every direction. Olber's Paradox disproves the Steady State hypothesis, it doesn't positively prove anything in particular. Popular treatments of Olber's Paradox are not obligated to come attached to a complete course in cosmology. I'm sure at least one of "Steven Soter and Neil deGrasse Tyson" are fully aware of all the issues raised in this entire HN page.
- goodside 11y ago
  There are plenty of ways the Universe could be steady-state and still not have a white-hot sky. Suppose you have metric expansion as you do now, but new matter magically appears as a sparse, uniform cloud of particles throughout space to feed the creation of new stars and maintain the energy density we see today. Metric expansion guarantees that light is redshifted in proportion to distance, so the sky remains dark, even though the Universe is infinitely old and stable. This model is wrong, obviously, but the reason has nothing to do with Olber's paradox.
  - jerf 11y ago
    The Steady State hypothesis isn't really a class of hypotheses, it was a specific one. The Steady State hypothesis predates the entire concept of metric expansion, after all. History, everyone. Olber's paradox is important because of the history, not its relevance to 21st century physics.
    - goodside 11y ago
      The model I described is the historical model, as developed (and later repudiated) by Einstein. It does not "predate the entire concept of metric expansion". The whole point was to reconcile expansion, which was both predicted by GR and confirmed by Hubble's observation of recessional redshift, with the belief that the Universe was infinitely old and unchanging. And there were many variants of Steady State, as the model was discovered independently after Einstein, and refined throughout the 1950s as its adherents struggled to explain new observations that distant quasars are far more common than near ones. There was even a brief (and apparently failed) attempt by Hoyle to revive a Steady State as late as 1993, involving widely varying rates of expansion in different regions of the Universe. What ultimately disproved Steady State was seeing the CMBR. None of this has anything to do with Olber's paradox, which is not an issue in a Steady State model for exactly the same reasons it's not an issue in modern cosmology.
Cyph0n 11y ago
So we don't know the age of the universe then?
- lutusp 11y ago
  We do know the age of the universe, to a reasonable certainty. The discussion above doesn't argue against our current estimate of the age of the universe -- it addresses other issues.
  - Cyph0n 11y ago
    So ~43 billion years is the age of the universe? Or is that extra 30 billion years solely a result of the universe's expansion?
    - hueving 11y ago
      43 billion light years is a distance, not a time. A star moving in the opposite direction of us that emitted light 15 billion years ago that we are just now seeing has moved a lot further away in that 15 billion years it took that initial light to reach us.
      - Cyph0n 11y ago
        I know what a light year is. I just did the conversion to time given that we are receiving a EM wave.
        xenophonf 11y ago
        It doesn't work like that. Yes, a photon travels at 299792458 m/s, but photons emitted 13 billion years ago (a redshift of about z=8.6) travel much further than (1310^9 years 3.1510^7 seconds/year 299792458 m/s) due to the metric expansion of space. The correct calculation involves an integral, an example of which you can see here: http://www.wolframalpha.com/widget/widgetPopup.jsp?p=v&id=453fadbd8a1a3af50a9df4df899537b5&title=Comoving%20distance%20%28Mpc%29%20from%20redshift%20%28flat%20LCDM%29&theme=red&i0=70.&i1=0.25&i2=8.6&podSelect=&includepodid=Result http://www.wolframalpha.com/widget/widgetPopup.jsp?p=v&id=45...
MAGZine 11y ago
It sort of is nitpicking. We haven't observed objects with redshifts that indicate distances larger than 13.some Gly away. Yes, the physical objects are farther, but the point is moot. Those objects aren't the same as what we see today. They're totally different. We don't know what they look like today. Even low-mass stars have lived and died. In summary, there is no way for us to see those 46.5 Gly objects other than wait 50 Gy (with our given technology)
- goodside 11y ago
  Yes, it's true that the most distant galaxies we've observed are only around 13 Gly away, but the fact that number is so close to (speed_of_light * age_of_universe) is just a coincidence. We could, in principle, see light emitted from points in space that are currently 46.5 Gly away, but all such light was emitted long before any galaxies had formed. The OP wasn't mentioning that we can see to a maximum distance of ~13 Gly as an unrelated fact with its own cause, it was saying it was because the Universe is ~13 Gy old and that's how far light can travel in that time, which isn't true.
  - goodside 11y ago
    Correction: I'm an idiot; see xenophonf's comment.
- xenophonf 11y ago
  I assume you're talking about UDFy-38135539 (https://en.wikipedia.org/wiki/UDFy-38135539 https://en.wikipedia.org/wiki/UDFy-38135539), with a redshift of z=8.6. It is not 13 Gly away, even though the light detected by Hubble's infrared camera was emitted about 13 billion years ago. Due to the metric expansion of space, the distance to that object is around 32 Gly (9710 Mpc according to http://www.bo.astro.it/~cappi/CosmoWidget.html http://www.bo.astro.it/~cappi/CosmoWidget.html).
  - MAGZine 11y ago
    that is one object with a suggested redshift of z=8.6. However, we do not know the distance of that object because not only was the image taken in the visible spectra (by Hubble), but the spectroscopic results re: it's distance have not been reproducible, and so the distance figure is challenged. Fortunately, we have other galaxies, such as EGSY8p7 (z=8.68) have been imaged in the infrared, and as such, their distance confirmed. However, you've just reiterated what GP has said. I maintain that nobody is denying that the object that has been observed is more distant than it was 13Gy ago, however, the light that we are currently observing emanated from an object that was only 13Gly away. As I stated in my previous comment, the galaxies that we are presently observing are much different, and much farther away today, it's just going to take a lot of time for us to see what they look like today. You're seeing a baby and claiming to have seen the senile crank that the baby potentially became over 19Gy. The baby we're currently observing may as well be dead. If the article author is stating that 13Gly is the maximum distance anything can be from us, then the author is false. However, that's not what the author is saying. The author is saying we cannot see past 13Gly because the light has not arrived, and this is patently TRUE. The author makes NO claim about the current location of the objects.
ank_the_elder 11y ago
But how can something that is further away than the maximum travel distance allowed by the time elapsed since the big bang exist? Is there a non-locality of the time dimension that makes it so? Was the expansion rate of the universe faster than the speed of light? Or... what?
- goodside 11y ago
  https://en.wikipedia.org/wiki/Metric_expansion_of_space https://en.wikipedia.org/wiki/Metric_expansion_of_space The short version: The distant object wasn't traveling through space, it sat stationary while more space came into existence in the gap between us and it, resulting in it being farther away. This happens everywhere, all the time -- it's what cosmologists are talking about when they say the Universe is "expanding". The laws that prohibit faster-than-light travel concern movement within space ("peculiar motion"), not the expansion of space itself.
- xenophonf 11y ago
  This might help you understand: "Misconceptions about the Big Bang" http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_BigBang.pdf http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_B...

kyberias 11y ago

> As spacetime expands, energy density decreases, space becomes less light-emitting and more transparent, and light already in transit becomes red-shifted. Our field of vision is still technically flooded with light, as it always has been, but by now the red-shift is so extreme that the light is in the radio spectrum. You talk about the radiation generated before 1M years in the universe. This article talked about the light from the existing stars in the universe.

goodside 11y ago
The red shift applies to distant stars too. The important point is that even in the extreme scenario where all stars last forever and you wait an infinite amount of time for light to reach you, you will still never see a fully illuminated sky because the stars are receding much faster than the light propagates due to metric expansion. Saying the sky is dark because 1) the Universe is young or 2) stars are short-lived is just false. The Universe gets darker and darker as it expands, despite there being "more time for light to reach us".
notalaser 11y ago
Yes, it's still incorrect! The complete quote is: > Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us. There may be an infinite number of stars beyond that cosmic horizon but we can’t see them because their light has not yet arrived. And the observable part of the universe contains too few stars to fill up the sky with light. (Emphasis mine because that's where the error is: 13 billion year-old light has traveled over a distance that is now much higher than than 13 billion light-years) Potential ambiguity (in cases such as this one) is one of the reasons why using light-years for distances may not be a good idea. (By the way, there are real astronomers arguing about this, too, e.g. http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html . So it's not just this nerd ranting on HN). GRB 090423 (admittedly, not a star -- it's a gamma ray burst) happened approximately 13 billion years ago. This means that light traveled for 13 billion years to reach us -- but, by the time it got to us, GRB 090423 was no longer 13 billion light-years from us, because all that space got bigger! GRB 090423 "was" about 9.2 Gpc away from us by the time that light reached us, i.e. about 30 billion light-years. If the light that hits us is 13 billion years old, that object (assuming it still exists) is a lot farther than 13 billion light-years from us now.
- vectorjohn 11y ago
  Sure. But that really is just pedantics. It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. You are essentially just giving those words the least charitable interpretation and then picking on that. It isn't wrong, its just not perfectly unambiguous.
  - notalaser 11y ago
    > It isn't wrong, its just not perfectly unambiguous. Yes, it's perfectly wrong, because those objects are not 13Gly away from us. A light-year really is defined as 300,000 km multiplied by whatever number of seconds there are in a year. The object is not at 13e9 x 300,000 x <a lot of seconds> kilometers from us if it took 13 billion years for the light to reach us. > It isn't a surprise to anyone to point out that the source of something that was moving away from us 13Gy ago is now further away than it was. If I wanted to give words the least charitable interpretation, I could point out that their motion has nothing to do with this -- they would still be farther than 13 Gly away from us even if they would not be moving away relative to the Earth :-).

joering2 11y ago

No, its correct. I lost a youtube video that explains Universe. Basically a universe observer is like an ant sitting on a big balloon that is constantly being pump with air. So while the balloon age might be 12 billion years old, the speed the air is pumped in it makes it grow and therefore objects "placed" far further than originally; hence 46B for an observer.

jerf 11y ago

At the risk of going more-pedantic-than-thou, this entire thread of nitpicks (and indeed, most of the posts I see right now to this story in general) is(/are) mostly irrelevant. The argument is not particularly sensitive to the exact way in which the universe is not steady state, including redshifting all the light until we're not staring at the sun in every direction. Olber's Paradox disproves the Steady State hypothesis, it doesn't positively prove anything in particular. Popular treatments of Olber's Paradox are not obligated to come attached to a complete course in cosmology. I'm sure at least one of "Steven Soter and Neil deGrasse Tyson" are fully aware of all the issues raised in this entire HN page.

goodside 11y ago
There are plenty of ways the Universe could be steady-state and still not have a white-hot sky. Suppose you have metric expansion as you do now, but new matter magically appears as a sparse, uniform cloud of particles throughout space to feed the creation of new stars and maintain the energy density we see today. Metric expansion guarantees that light is redshifted in proportion to distance, so the sky remains dark, even though the Universe is infinitely old and stable. This model is wrong, obviously, but the reason has nothing to do with Olber's paradox.
- jerf 11y ago
  The Steady State hypothesis isn't really a class of hypotheses, it was a specific one. The Steady State hypothesis predates the entire concept of metric expansion, after all. History, everyone. Olber's paradox is important because of the history, not its relevance to 21st century physics.
  - goodside 11y ago
    The model I described is the historical model, as developed (and later repudiated) by Einstein. It does not "predate the entire concept of metric expansion". The whole point was to reconcile expansion, which was both predicted by GR and confirmed by Hubble's observation of recessional redshift, with the belief that the Universe was infinitely old and unchanging. And there were many variants of Steady State, as the model was discovered independently after Einstein, and refined throughout the 1950s as its adherents struggled to explain new observations that distant quasars are far more common than near ones. There was even a brief (and apparently failed) attempt by Hoyle to revive a Steady State as late as 1993, involving widely varying rates of expansion in different regions of the Universe. What ultimately disproved Steady State was seeing the CMBR. None of this has anything to do with Olber's paradox, which is not an issue in a Steady State model for exactly the same reasons it's not an issue in modern cosmology.

Cyph0n 11y ago

So we don't know the age of the universe then?

lutusp 11y ago
We do know the age of the universe, to a reasonable certainty. The discussion above doesn't argue against our current estimate of the age of the universe -- it addresses other issues.
- Cyph0n 11y ago
  So ~43 billion years is the age of the universe? Or is that extra 30 billion years solely a result of the universe's expansion?
  - hueving 11y ago
    43 billion light years is a distance, not a time. A star moving in the opposite direction of us that emitted light 15 billion years ago that we are just now seeing has moved a lot further away in that 15 billion years it took that initial light to reach us.
    - Cyph0n 11y ago
      I know what a light year is. I just did the conversion to time given that we are receiving a EM wave.
      - xenophonf 11y ago
        It doesn't work like that. Yes, a photon travels at 299792458 m/s, but photons emitted 13 billion years ago (a redshift of about z=8.6) travel much further than (1310^9 years 3.1510^7 seconds/year 299792458 m/s) due to the metric expansion of space. The correct calculation involves an integral, an example of which you can see here: http://www.wolframalpha.com/widget/widgetPopup.jsp?p=v&id=453fadbd8a1a3af50a9df4df899537b5&title=Comoving%20distance%20%28Mpc%29%20from%20redshift%20%28flat%20LCDM%29&theme=red&i0=70.&i1=0.25&i2=8.6&podSelect=&includepodid=Result http://www.wolframalpha.com/widget/widgetPopup.jsp?p=v&id=45...

MAGZine 11y ago

It sort of is nitpicking. We haven't observed objects with redshifts that indicate distances larger than 13.some Gly away. Yes, the physical objects are farther, but the point is moot. Those objects aren't the same as what we see today. They're totally different. We don't know what they look like today. Even low-mass stars have lived and died. In summary, there is no way for us to see those 46.5 Gly objects other than wait 50 Gy (with our given technology)

goodside 11y ago
Yes, it's true that the most distant galaxies we've observed are only around 13 Gly away, but the fact that number is so close to (speed_of_light * age_of_universe) is just a coincidence. We could, in principle, see light emitted from points in space that are currently 46.5 Gly away, but all such light was emitted long before any galaxies had formed. The OP wasn't mentioning that we can see to a maximum distance of ~13 Gly as an unrelated fact with its own cause, it was saying it was because the Universe is ~13 Gy old and that's how far light can travel in that time, which isn't true.
- goodside 11y ago
  Correction: I'm an idiot; see xenophonf's comment.
xenophonf 11y ago
I assume you're talking about UDFy-38135539 (https://en.wikipedia.org/wiki/UDFy-38135539 https://en.wikipedia.org/wiki/UDFy-38135539), with a redshift of z=8.6. It is not 13 Gly away, even though the light detected by Hubble's infrared camera was emitted about 13 billion years ago. Due to the metric expansion of space, the distance to that object is around 32 Gly (9710 Mpc according to http://www.bo.astro.it/~cappi/CosmoWidget.html http://www.bo.astro.it/~cappi/CosmoWidget.html).
- MAGZine 11y ago
  that is one object with a suggested redshift of z=8.6. However, we do not know the distance of that object because not only was the image taken in the visible spectra (by Hubble), but the spectroscopic results re: it's distance have not been reproducible, and so the distance figure is challenged. Fortunately, we have other galaxies, such as EGSY8p7 (z=8.68) have been imaged in the infrared, and as such, their distance confirmed. However, you've just reiterated what GP has said. I maintain that nobody is denying that the object that has been observed is more distant than it was 13Gy ago, however, the light that we are currently observing emanated from an object that was only 13Gly away. As I stated in my previous comment, the galaxies that we are presently observing are much different, and much farther away today, it's just going to take a lot of time for us to see what they look like today. You're seeing a baby and claiming to have seen the senile crank that the baby potentially became over 19Gy. The baby we're currently observing may as well be dead. If the article author is stating that 13Gly is the maximum distance anything can be from us, then the author is false. However, that's not what the author is saying. The author is saying we cannot see past 13Gly because the light has not arrived, and this is patently TRUE. The author makes NO claim about the current location of the objects.

ank_the_elder 11y ago

But how can something that is further away than the maximum travel distance allowed by the time elapsed since the big bang exist? Is there a non-locality of the time dimension that makes it so? Was the expansion rate of the universe faster than the speed of light? Or... what?

goodside 11y ago
https://en.wikipedia.org/wiki/Metric_expansion_of_space https://en.wikipedia.org/wiki/Metric_expansion_of_space The short version: The distant object wasn't traveling through space, it sat stationary while more space came into existence in the gap between us and it, resulting in it being farther away. This happens everywhere, all the time -- it's what cosmologists are talking about when they say the Universe is "expanding". The laws that prohibit faster-than-light travel concern movement within space ("peculiar motion"), not the expansion of space itself.
xenophonf 11y ago
This might help you understand: "Misconceptions about the Big Bang" http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_BigBang.pdf http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_B...