<p>The question has been addressed, but for completeness I should like to remark that the most thorough and readily understandable discussion of Olber's Paradox is that of E. Harrison in his book <em>Cosmology, the Science of the Universe</em> (CUP 2000).  </p>

<p>It is also worth remarking that we now know that the sky at night is not in fact dark: the light from the cosmic microwave background (CMB) dominates all other sources of radiation, including the sum total of all the stars.  Most of the CMB radiation comes out in the microwave region of the electromagnetic spectrum.  This was discussed by M. Longair  and R.A. Sunyaev in the journal <em>Astrophysical Letters</em> (vol 4 pp65-70 1969) with the following sketch<a href="https://i.sstatic.net/S0aRt.jpg" rel="noreferrer"><img src="https://i.sstatic.net/S0aRt.jpg" alt="The intensity of the radiation received from the sky over all of the electromagnetic spectrum.  The dashed lines were tentative estimates when this was first drawn.  The flux of the 2.7 CMB is shown as a blackbody dominating all other wavelengths."></a></p>

<p>Updates on this were published in 1990 by M. Ressell and M. Turner (<em>Comments on Astrophysics</em>, vol 14 p323) and by R. Henry in 1999 (<em>Astrophys. J.</em> Vol 516 pp. L49-L52).  </p>

<p>So the sky is not dark at night.  The light of the night sky is due to the CMB and not due to stars.  The finite lifetime of stars, the finite age of the universe and the cosmic expansion have together reduced the contribution from starlight. </p>

CC BY-SA 3.0
2017-08-01T22:09:34.390Z
2017-08-01T22:09:34.390Z
JonesTheAstronomer
<p>Olber's Paradox was created at a time before the idea of a finite universe was accepted.  (<a href="https://en.wikipedia.org/wiki/Olbers%27_paradox#History" rel="noreferrer">It was thought of in the 1600's</a>).  In order to resolve Olber's Paradox, you have to introduce the idea that either the universe had a beginning or it is of finite size.  (Note: the solution does not require an expanding universe).  So, at the time, it was a paradox.  Pretty much all astronomers considered the universe to be static and infinite.  Therefore, the fact that their observations didn't fit with what they expected made it a paradox.</p>

CC BY-SA 3.0
2017-07-28T06:14:02.327Z
Note. It was first suggested in the 1600s, but not by Obler. When first suggested, the universe was thought to be finite in time, but it was uncertain whether light travelled at an infinite speed.
James K
CC BY-SA 3.0
2017-07-28T09:44:43.810Z
@JamesK Do you have references to support your assertion?
FKEinternet
CC BY-SA 3.0
2017-07-28T10:22:36.000Z
[Obler was born 1758](https://en.wikipedia.org/wiki/Olbers%27_paradox), and published in 1823. Kepler stated the problem in 1610. [Descarte seems ambivalent](http://www.springer.com/cda/content/document/cda_downloaddocument/9781461406150-c1.pdf) on the speed of light, saying that it moves instantly, but later explaining refraction in terms of the acceleration of light. The first measurement of the speed of light was by [Romer](https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light) in 1676. That the universe had a beginning is Gen 1:1. All these men were Christian.
James K
CC BY-SA 3.0
2017-07-28T18:18:15.443Z
@JamesK It's Olbers, not Obler.
hobbs
CC BY-SA 3.0
2017-07-28T21:35:06.340Z
Couldn't you also have an eternal though finite universe to fix the problem?
k_g
CC BY-SA 3.0
2017-07-29T03:59:43.533Z
@phiteros wouldn't it be 13 billion light years in radius? Or am I missing something?
k_g
CC BY-SA 3.0
2017-07-29T05:11:54.697Z
@k_g I am looking over my notes on the matter and I got one of the equations wrong, so I'm not sure what the exact number is, but it is larger than I thought.  You can have a finite universe, but you do have to be careful about the size, because even if you have a small universe, given enough time, every line of sight will end on a star.
Phiteros
CC BY-SA 3.0
2017-07-31T20:35:23.283Z
Just a clarification: to my understanding, as astronomers we have not settled on whether the universe is finite or infinite. We simply don't know (yet), and using current cosmological models, Olber's paradox is resolved in either case.
Néstor
CC BY-SA 3.0
2017-07-31T21:23:21.813Z
@Néstor Olber's paradox is resolved by the fact that the universe had a beginning.  So it doesn't matter whether the universe is finite or infinite in this case.
Phiteros
CC BY-SA 3.0
2017-08-01T21:51:41.117Z
@Phiteros Exactly; I just wanted to point out that the finiteness of the universe, today, couldn't be an answer, as we don't know if the universe is or not finite. The finiteness of the age of the universe (or, at the very least, the finiteness of the appearance of the first stars) is the resolution of the Paradox.
Néstor
CC BY-SA 3.0
2017-08-03T03:10:26.327Z
The *currently accepted **theory*** is that the "universe" began with a "big bang" 14 billion years ago, plus or minus a billion or so - although that number seems to have a tendency to grow.  The truth is *the sky **is** white* - when you're looking in the microwave range.  *[My own theory](http://theskyiswhite.org/)* is that there's enough *dark* matter (i.e., not intrinsically radiating energy) between us and most of the *infinite* universe so that there's effectively a curtain between us and most of the energy that's *out there*.
FKEinternet
CC BY-SA 3.0
2017-08-03T04:58:59.260Z
@FKEinternet
A: Dark matter does not interact with EM radiation, so it would not observe anyways.
B: The CMB is fading heat left over from the big bang, not light from stars proposed by Olber.
C: The reason we can't see past a certain radius is because light has not had enough time to reach us yet.  Light travels at a finite speed, and the universe had a beginning.
Phiteros
CC BY-SA 3.0
2017-08-03T06:21:26.157Z
@Phiteros As I said, the *currently accepted **theory*** is the universe had a "big bang" beginning - but since neither you nor I was there, all we have is that *theory*, generally assumed to "prove" that the CMB is the big bang's shadow, and more distant light hasn't had time to reach us.  Your "dark matter" (a theoretical substance no one can explain) is not the "dark matter" I referred to (random atoms and dust dispersed throughout the universe that don't emit light on their own).  The latter will re-radiate once enough photons bombard it, probably at a lower temperature - perhaps the CMB?
FKEinternet
CC BY-SA 3.0
2017-08-03T15:18:13.903Z
@FKEinternet Alright, there are multiple problems with your statement, but this answer's comments are not the place to discuss it.
Phiteros
CC BY-SA 3.0
2017-07-28T05:46:06.057Z
2017-07-29T05:13:00.973Z
2017-07-29T05:13:00.973Z
Phiteros
<p>To the extent of my understanding, Olber’s paradox states that if the universe was static and homogeneous, we should see a star at every point in the night sky and therefore the night sky should be equally as bright as day.</p>

<p>However, since the night sky is dark and non-uniform, it can be said that the universe is <em>not</em> static and <em>not</em> homogeneous. However, if this was already known, what exactly is the “paradox”? Why isn’t it called Olber’s Observation or something else?</p>

CC BY-SA 3.0
2017-07-28T08:54:17.803Z
Its the same paradox as what I asked me in the first grade of low school, when our teacher told as universe is infinite. I asked my self, that would mean there are infinite planets where an infinite subset of them will for sure host living species where again infinite species can run space projects resulting in infinite arrival of alien artifacts/species on our planet. So I just thought for many years he can't be right! I developed my own view of whats the universe and within the last years I realized I was closer what science thinks about the universe, then what he was. '^.^
Zaibis
CC BY-SA 3.0
2017-07-28T11:07:10.377Z
Static, homogeneous and INFINITE. These were the known (accepted) facts about the universe at that time. Yet the night sky was mostly dark. The facts can't be wrong (so everybody thought), so what else?
Klaws
CC BY-SA 3.0
2017-07-29T01:04:37.790Z
@Zaibis infinte planets and infinite subset of them with space does _not_ result in an infinite arrival species, as that last bit is a factor of density. If we assume they're all limited by the speed of light, and the age of the universe, even with infinite space faring aliens we're not guaranteed to see any evidence due to low density.
Mooing Duck
CC BY-SA 3.0
2017-08-03T15:57:30.273Z
The same holds for Olber paradox
Alchimista
CC BY-SA 4.0
2024-03-06T18:13:55.513Z
The universe can easily be static, homogeneous and infinite and the night sky still be dark given an observation documented more than 100 years after Olbers posed his "paradox":
https://en.wikipedia.org/wiki/Extinction_(astronomy)
pygosceles
<star><universe><history><night-sky>
What exactly is the "paradox" in Olber's Paradox?
Sam
# What exactly is the "paradox" in Olber's Paradox?

(Asked by: username_0 on 2017-07-28)

To the extent of my understanding, Olber’s paradox states that if the universe was static and homogeneous, we should see a star at every point in the night sky and therefore the night sky should be equally as bright as day.

However, since the night sky is dark and non-uniform, it can be said that the universe is *not* static and *not* homogeneous. However, if this was already known, what exactly is the “paradox”? Why isn’t it called Olber’s Observation or something else?

**Tags:** star, universe, history, night-sky

> **Comments:**
> **username_1**: Its the same paradox as what I asked me in the first grade of low school, when our teacher told as universe is infinite. I asked my self, that would mean there are infinite planets where an infinite subset of them will for sure host living species where again infinite species can run space projects resulting in infinite arrival of alien artifacts/species on our planet. So I just thought for many years he can't be right! I developed my own view of whats the universe and within the last years I realized I was closer what science thinks about the universe, then what he was. '^.^  
> **username_2**: Static, homogeneous and INFINITE. These were the known (accepted) facts about the universe at that time. Yet the night sky was mostly dark. The facts can't be wrong (so everybody thought), so what else?  
> **username_3**: @username_1 infinte planets and infinite subset of them with space does _not_ result in an infinite arrival species, as that last bit is a factor of density. If we assume they're all limited by the speed of light, and the age of the universe, even with infinite space faring aliens we're not guaranteed to see any evidence due to low density.  
> **username_4**: The same holds for Olber paradox  
> **username_5**: The universe can easily be static, homogeneous and infinite and the night sky still be dark given an observation documented more than 100 years after Olbers posed his "paradox": https://en.wikipedia.org/wiki/Extinction_(astronomy)  

---

## 2 answers

### Answer by username_7 on 2017-07-28. Score: 30 | Accepted answer

Olber's Paradox was created at a time before the idea of a finite universe was accepted. ([It was thought of in the 1600's](https://en.wikipedia.org/wiki/Olbers%27_paradox#History)). In order to resolve Olber's Paradox, you have to introduce the idea that either the universe had a beginning or it is of finite size. (Note: the solution does not require an expanding universe). So, at the time, it was a paradox. Pretty much all astronomers considered the universe to be static and infinite. Therefore, the fact that their observations didn't fit with what they expected made it a paradox.

> **Comments:**
> **username_8**: Note. It was first suggested in the 1600s, but not by Obler. When first suggested, the universe was thought to be finite in time, but it was uncertain whether light travelled at an infinite speed.  
> **username_9**: @username_8 Do you have references to support your assertion?  
> **username_8**: [Obler was born 1758](https://en.wikipedia.org/wiki/Olbers%27_paradox), and published in 1823. Kepler stated the problem in 1610. [Descarte seems ambivalent](http://www.springer.com/cda/content/document/cda_downloaddocument/9781461406150-c1.pdf) on the speed of light, saying that it moves instantly, but later explaining refraction in terms of the acceleration of light. The first measurement of the speed of light was by [Romer](https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light) in 1676. That the universe had a beginning is Gen 1:1. All these men were Christian.  
> **username_10**: @username_8 It's Olbers, not Obler.  
> **username_11**: Couldn't you also have an eternal though finite universe to fix the problem?  
> **username_11**: @username_7 wouldn't it be 13 billion light years in radius? Or am I missing something?  
> **username_7**: @username_11 I am looking over my notes on the matter and I got one of the equations wrong, so I'm not sure what the exact number is, but it is larger than I thought.  You can have a finite universe, but you do have to be careful about the size, because even if you have a small universe, given enough time, every line of sight will end on a star.  
> **username_12**: Just a clarification: to my understanding, as astronomers we have not settled on whether the universe is finite or infinite. We simply don't know (yet), and using current cosmological models, Olber's paradox is resolved in either case.  
> **username_7**: @username_12 Olber's paradox is resolved by the fact that the universe had a beginning.  So it doesn't matter whether the universe is finite or infinite in this case.  
> **username_12**: @username_7 Exactly; I just wanted to point out that the finiteness of the universe, today, couldn't be an answer, as we don't know if the universe is or not finite. The finiteness of the age of the universe (or, at the very least, the finiteness of the appearance of the first stars) is the resolution of the Paradox.  
> **username_9**: The *currently accepted **theory*** is that the "universe" began with a "big bang" 14 billion years ago, plus or minus a billion or so - although that number seems to have a tendency to grow.  The truth is *the sky **is** white* - when you're looking in the microwave range.  *[My own theory](http://theskyiswhite.org/)* is that there's enough *dark* matter (i.e., not intrinsically radiating energy) between us and most of the *infinite* universe so that there's effectively a curtain between us and most of the energy that's *out there*.  
> **username_7**: @username_9 A: Dark matter does not interact with EM radiation, so it would not observe anyways. B: The CMB is fading heat left over from the big bang, not light from stars proposed by Olber. C: The reason we can't see past a certain radius is because light has not had enough time to reach us yet.  Light travels at a finite speed, and the universe had a beginning.  
> **username_9**: @username_7 As I said, the *currently accepted **theory*** is the universe had a "big bang" beginning - but since neither you nor I was there, all we have is that *theory*, generally assumed to "prove" that the CMB is the big bang's shadow, and more distant light hasn't had time to reach us.  Your "dark matter" (a theoretical substance no one can explain) is not the "dark matter" I referred to (random atoms and dust dispersed throughout the universe that don't emit light on their own).  The latter will re-radiate once enough photons bombard it, probably at a lower temperature - perhaps the CMB?  
> **username_7**: @username_9 Alright, there are multiple problems with your statement, but this answer's comments are not the place to discuss it.  

---

### Answer by username_6 on 2017-08-01. Score: 9

The question has been addressed, but for completeness I should like to remark that the most thorough and readily understandable discussion of Olber's Paradox is that of E. Harrison in his book *Cosmology, the Science of the Universe* (CUP 2000).

It is also worth remarking that we now know that the sky at night is not in fact dark: the light from the cosmic microwave background (CMB) dominates all other sources of radiation, including the sum total of all the stars. Most of the CMB radiation comes out in the microwave region of the electromagnetic spectrum. This was discussed by M. Longair and R.A. Sunyaev in the journal *Astrophysical Letters* (vol 4 pp65-70 1969) with the following sketch[![The intensity of the radiation received from the sky over all of the electromagnetic spectrum.  The dashed lines were tentative estimates when this was first drawn.  The flux of the 2.7 CMB is shown as a blackbody dominating all other wavelengths.](https://i.sstatic.net/S0aRt.jpg)](https://i.sstatic.net/S0aRt.jpg)

Updates on this were published in 1990 by M. Ressell and M. Turner (*Comments on Astrophysics*, vol 14 p323) and by R. Henry in 1999 (*Astrophys. J.* Vol 516 pp. L49-L52).

So the sky is not dark at night. The light of the night sky is due to the CMB and not due to stars. The finite lifetime of stars, the finite age of the universe and the cosmic expansion have together reduced the contribution from starlight.


---

