Olbers's paradox
Olbers's paradox, also known as the dark night paradox, is an argument in
The darkness of the night sky is one piece of evidence for a dynamic universe, such as the
Other explanations for the paradox have been offered, but none have wide acceptance in cosmology. Although he was not the first to describe it, the paradox is popularly named after the German astronomer Heinrich Wilhelm Olbers (1758–1840).
History
The first one to address the problem of an infinite number of stars and the resulting heat in the Cosmos was
Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy – since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.[6]
The paradox
The paradox is that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark.[1]
To show this, we divide the universe into a series of concentric shells, 1 light year thick. A certain number of stars will be in the shell, say, 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 and 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear one quarter as bright as the stars in the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell.
Thus each shell of a given thickness will produce the same net amount of light regardless of how far away it is. That is, the light of each shell adds to the total amount. Thus the more shells, the more light; and with infinitely many shells, there would be a bright night sky.
While dark clouds could obstruct the light, these clouds would heat up, until they were as hot as the stars, and then radiate the same amount of light.
Kepler saw this as an argument for a finite
Explanation
The poet
However, the Big Bang theory seems to introduce a new problem: it states that the sky was much brighter in the past, especially at the end of the recombination era, when it first became transparent. All points of the local sky at that era were comparable in brightness to the surface of the Sun, due to the high temperature of the universe in that era; and most light rays will originate not from a star but the relic of the Big Bang.
This problem is addressed by the fact that the Big Bang theory also involves the
Other factors
Steady state
The redshift hypothesised in the Big Bang model would by itself explain the darkness of the night sky even if the universe were infinitely old. In the
e.g. for temperature 2.7 K it is 40 fJ/m3 ... 4.5×10−31 kg/m3 and for visible temperature 6000 K we get 1 J/m3 ... 1.1×10−17 kg/m3. But the total radiation emitted by a star (or other cosmic object) is at most equal to the total nuclear binding energy of isotopes in the star. For the density of the observable universe of about 4.6×10−28 kg/m3 and given the known abundance of the chemical elements, the corresponding maximal radiation energy density of 9.2×10−31 kg/m3, i.e. temperature 3.2 K (matching the value observed for the optical radiation temperature by Arthur Eddington[11][12]). This is close to the summed energy density of the cosmic microwave background (CMB) and the cosmic neutrino background. However, the steady-state model does not predict the angular distribution of the microwave background temperature accurately (as the standard ΛCDM paradigm does).[13]
Brightness
Suppose that the universe were not expanding, and always had the same stellar density; then the temperature of the universe would continually increase as the stars put out more radiation. Eventually, it would reach 3000 K (corresponding to a typical photon energy of 0.3
Fractal star distribution
A different resolution, which does not rely on the Big Bang theory, was first proposed by
Mathematically, the light received from stars as a function of star distance in a hypothetical fractal cosmos is[citation needed]
where:
- r0 = the distance of the nearest star, r0 > 0;
- r = the variable measuring distance from the Earth;
- L(r) = average luminosity per star at distance r;
- N(r) = number of stars at distance r.
The function of luminosity from a given distance L(r)N(r) determines whether the light received is finite or infinite. For any luminosity from a given distance L(r)N(r) proportional to ra, is infinite for a ≥ −1 but finite for a < −1. So if L(r) is proportional to r−2, then for to be finite, N(r) must be proportional to rb, where b < 1. For b = 1, the numbers of stars at a given radius is proportional to that radius. When integrated over the radius, this implies that for b = 1, the total number of stars is proportional to r2. This would correspond to a fractal dimension of 2. Thus the fractal dimension of the universe would need to be less than 2 for this explanation to work.
This explanation is not widely accepted among cosmologists, since the evidence suggests that the
See also
References
- ^ a b c Overbye, Dennis (3 August 2015). "The Flip Side of Optimism About Life on Other Planets". The New York Times. Retrieved 29 October 2015.
- ^ "Cosmas Indicopleustès. Topographie chrétienne, 3 vols.", Ed. Wolska–Conus, W.Paris: Cerf, 1:1968; 2:1970; 3:1973; Sources chrétiennes, Book 10, section 27, line 7 "Cosmas Indicopleustès. Topographia Christiana (4061: 002) Topographie chrétienne, 3 vols.", Ed. Wolska–Conus, W. Paris: Cerf, 1:1968; 2:1970; 3:1973; Sources chrétiennes 141, 159, 197. Book 10, section 27, line 7 (Κρυσταλλώδης ἦν ὁ οὐρανὸς ἀπὸ ὑδάτων παγείς· ἐπειδὴ δὲ ἔμελλε δέχεσθαι ἡλίου φλόγα καὶ σελήνης καὶ ἄστρων ἄπειρα πλήθη, καὶ ἦν ὅλος πυρὸς πεπληρωμένος, ἵνα μὴ οὕτως ὑπὸ τῆς θερμότητος λυθῇ ἢ φλεχθῇ ἄστρων ἄπειρα πλήθη, καὶ ἦν ὅλος πυρὸς πεπληρωμένος, ἵνα μὴ οὕτως ὑπὸ τῆς θερμότητος λυθῇ ἢ φλεχθῇ.)
- ISBN 9780470754771.
The Puritan Thomas Digges (1546–1595?) was the earliest Englishman to offer a defense of the Copernican theory. ... Accompanying Digges's account is a diagram of the universe portraying the heliocentric system surrounded by the orb of fixed stars, described by Digges as infinitely extended in all dimensions.
- ^ ISBN 9783540678779.
The simple observation that the night sky is dark allows far-reaching conclusions to be drawn about the large-scale structure of the universe. This was already realized by J. Kepler (1610), E. Halley (1720), J.-P. Loy de Chesaux (1744), and H. W. M. Olbers (1826).
- ^ For a key extract from this paper, see Harrison (1987), pp. 227–28.
- ^ Poe, Edgar Allan (1848). "Eureka: A Prose Poem". Archived from the original on 26 April 2008.
- ISBN 9780198596868.
- ^ "Poe: Eureka". Xroads.virginia.edu. Retrieved 9 May 2013.
- ^ "Brief Answers to Cosmic Questions". Universe Forum. Retrieved 27 January 2023 – via harvard.edu.
- ISBN 3110258781.
- ^ Wright, Edward L. (23 October 2006). "Eddington's Temperature of Space". Retrieved 10 July 2013.
- ^ Eddington, A.S. (1926). Eddington's 3.18K "Temperature of Interstellar Space". Cambridge University Press. pp. 371–372. Retrieved 10 July 2013.
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ignored (help) - ^ Wright, E. L., E. L. "Errors in the Steady State and Quasi-SS Models". UCLA, Physics and Astronomy Department. Retrieved 28 May 2015.
- S2CID 17424588.
- S2CID 14466810.
- S2CID 15259697.
- S2CID 15957886.
Further reading
- ISBN 9780674192713.
- ISBN 9781009215701.
- Wesson, Paul (1991). "Olbers' paradox and the spectral intensity of the extragalactic background light". doi:10.1086/169638.
- Zamarovský, Peter (2013). Why is it Dark at Night? Story of Dark Night Sky Paradox. AuthorHouseUK. ISBN 978-1491878804.
External links
- Relativity FAQ about Olbers's paradox
- Astronomy FAQ about Olbers's paradox
- Cosmology FAQ about Olbers's paradox
- "On Olber's Paradox". MathPages.com.
- Why is the sky dark? physics.org page about Olbers's paradox
- Why is it dark at night? A 60-second animation from the Perimeter Instituteexploring the question with Alice and Bob in Wonderland