It is generally believed that in the epoch prior to the formation of the first stars, the Universe was completely dark (the period is therefore known as the Dark Ages). Usually, the start of this epoch is placed at photon decoupling. In this work, we investigate the question whether there was enough light during the dark epoch for a human eye to see. We use the black body spectrum of the Universe to find the flux of photon energy for different temperatures and compare them with visual limits of brightness and darkness. We find that the Dark Ages actually began approximately 6 × 106 years later than commonly stated.

1.
Edward W.
Kolb
and
Michael S.
Turner
,
The Early Universe
, 1st ed. (
Addison-Wesley Publishing Company
,
Redwood City
,
1988
). pp.
70
82
.
2.
Barbara
Ryden
,
Introduction to Cosmology
, 1st ed. (
Pearson Education Limited
,
Harlow
,
2006
).
3.
Sujan
Sengupta
,
Worlds Beyond Our Own: The Search for Habitable Planets
, 1st ed. (
Springer
,
New York
,
2015
). p.
5
.
4.
Volker
Bromm
, “
Formation of the first stars
,”
Rep. Prog. Phys.
76
(
11
),
112901
(
2013
); e-print arXiv:1305.5178 [astro-ph.CO].
5.
Judd D.
Bowman
 et al, “
An absorption profile centred at 78 megahertz in the sky-averaged spectrum
,”
Nature
555
,
67
70
(
2018
).
6.
J.
Miralda-Escude
, “
The Dark Age of the Universe
,”
Science
300
(
5627
),
1904
1909
(
2003
); e-print arXiv:astro-ph/0307396.
7.
Andrew J. S.
Hamilton
and
Gavin
Polhemus
, “
The edge of locality: Visualizing a black hole from the inside
,” e-print arXiv:0903.4717 [gr-qc].
8.
S.
Krasnikov
, “
Falling into the Schwarzschild black hole. Important details
,”
Gravit. Cosmol.
14
(
4
),
362
367
(2008); e-print arXiv:0804.3619 [gr-qc].
9.
A. A.
Grib
and
Yu. V.
Pavlov
, “
Is it possible to see the infinite future of the Universe when falling into a black hole?
,” Usp. Fiz. Nauk 173(3), 279–283 (
2009
) (in Russian) [
Phys. Usp.
52
(
3
),
257
261
(
2009
)]; e-print arXiv:0906.1442 [gr-qc].
10.
K.
Nagamine
and
A.
Loeb
, “
Future evolution of the intergalactic medium in a universe dominated by a cosmological constant
,”
New Astron.
9
(
8
),
573
583
(
2004
);
K.
Nagamine
and
A.
Loeb
, e-print arXiv:astro-ph/0310505.
11.
S. P.
Kashyap
 et al, “
Surviving in a metastable de sitter space-time
,”
J. High Energy Phys.
2015
(
9
),
139
(
2015
); e-print arXiv:1506.00772 [hep-th].
12.
A.
Loeb
, “
The optimal cosmic epoch for precision cosmology
,”
J. Cosmol. Astropart. P.
2012
(
05
),
028
(
2012
); e-print arXiv:1203.2622 [astro-ph.CO].
13.
A.
Loeb
, “
Cosmology with hypervelocity stars
,”
J. Cosmol. Astropart. P.
2011
(
04
),
023
(
2011
); e-print arXiv:1102.0007 [astro-ph.CO].
14.
L. M.
Krauss
and
G. D.
Starkman
, “
Life, the universe, and nothing: Life and death in an ever-expanding universe
,”
Astrophys. J.
531
,
22
30
(
2000
); e-print arXiv:astro-ph/9902189.
15.
L. M.
Krauss
and
R. J.
Scherrer
, “
The end of cosmology?
,”
Sci. Am.
298
(
3
),
34
41
(
2008
).
16.
L. M.
Krauss
, “
The Big Bang, modern cosmology and the fate of the Universe: impacts upon culture
,”
IAU Symp.
260
,
33
38
(
2011
); e-print arXiv:1002.1952 [astro-ph.CO].
17.
L. M.
Krauss
and
R. J.
Scherrer
, “
The return of a static universe and the end of cosmology
,”
Gen. Relat. Gravit.
39
,
1545
1550
(
2007
);
L. M.
Krauss
and
R. J.
Scherrer
,
Int. J. Mod. Phys. D
17
,
685
690
(
2008
); e-print arXiv:0704.0221 [astro-ph].
18.
T.
Opatrný
,
L.
Richterek
, and
P.
Bakala
, “
Life under a black sun
,”
Am. J. Phys.
85
(
1
),
14
22
(
2017
); e-print arXiv:1601.02897 [gr-qc].
19.
K. T.
Smith
, “
GRBs not fatal for life in early universe
,”
Science
350
(
6256
),
53
54
(
2015
).
20.
Z.
Merali
, “
Life possible in the early Universe
,”
Nature
504
(
7479
),
201
(
2013
).
21.
A.
Loeb
, “
The habitable epoch of the early Universe
,”
Int. J. Astrobiol.
13
(
4
),
337
339
(
2014
); e-print arXiv:1312.0613 [astro-ph.CO].
22.
A.
Loeb
, “
On the Habitability of Our Universe
,” e-print arXiv:1606.08926 [astro-ph.CO].
23.
Y.
Li
and
B.
Zhang
, “
Can life survive gamma-ray bursts in the high-redshift universe?
,”
Astrophys. J.
810
(
1
),
41
(
2015
); e-print arXiv:1507.05966 [astro-ph.HE].
24.
S. J.
Olson
, “
Life-hostile conditions in the early universe can increase the present-day odds of observing extragalactic life
,” e-print arXiv:1704.04125 [physics.pop-ph].
25.
L. M.
Krauss
, “
Cosmology as seen from Venice
,” in
Neutrino Telescopes, Proceedings, 9th International Workshop, Venice, Italy, March 6-9 (2001)
, Vol.
1, 2
, pp.
673
689
(
2001
); e-print arXiv:astro-ph/0106149.
26.
M. S.
Morris
and
K. S.
Thorne
, “
Wormholes in space-time and their use for interstellar travel: A tool for teaching general relativity
,”
Am. J. Phys.
56
,
395
412
(
1988
).
27.
T.
Muller
, “
Visual appearance of a Morris-Thorne-wormhole
,”
Am. J. Phys.
72
(a),
1045
1050
(
2004
).
28.
O.
James
 et al, “
Visualizing Interstellar's Wormhole
,”
Am. J. Phys.
83
(
6
),
486
499
(
2015
); e-print arXiv:1502.03809 [gr-qc].
29.
O.
James
 et al, “
Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar
,”
Classical Quant. Grav.
32
(
06
),
065001
(
2015
); <http://stacks.iop.org/0264-9381/32/i=6/a=065001>.
30.
K.
Thorne
,
The Science of Interstellar
(
W. W. Norton & Company
,
New York
,
2014
).
31.
E.
Fred Schubert
,
Light-Emitting Diodes
, 2nd ed. (
University Cambridge Press
,
Cambridge
,
2006
),
pp. 275
276
, 283, and 290–291.
32.
Robert
Snowden
,
Peter
Thompson
, and
Tom
Troscianko
,
Basic Vision: An Introduction to Visual Perception
, revised edition (
Oxford U.P.
,
Oxford
,
2006
),
pp. 20
25
and 47.
33.
Malgorzata
Rozanowska
,
Bartosz
Rozanowski
, and
Michael
Boulton
, “
Light-induced damage to the retina
,”
Photochem. Photobiol.
88
(
6
),
1303
1308
(
2012
).
34.
Jonathan M.
Marr
and
Francis P.
Wilkin
, “
A better presentation of Planck's radiation law
,”
Am. J. Phys.
80
(
5
),
399
405
(
2012
).
35.
Ian
Ashdown
,
Radiosity: A Programmer's Perspective
(
John Wiley and Sons
,
New York
,
1994
), pp.
483
486
.
36.
M. S.
Bessell
,
F.
Castelli
, and
B.
Plez
, “
Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O-M stars
,”
Astron. Astrophys.
333
,
231
250
(
1998
).
37.
Information concerning the Sun can be found at the NSSDC website under Fact Sheets, <https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html>.
38.
S.
Hecht
,
S.
Shlaer
, and
M. H.
Pirenne
, “
Energy, quanta, and vision
,”
J. Gen. Physiol.
25
,
819
840
(
1942
).
39.
J.
Chariton
and
C. A.
Lea
, “
Some experiments concerning the counting of scintillations produced by alpha particles
,”
Proc. Roy. Soc. A.
122
,
304
319
(
1929
).
40.
S. P.
Langley
, “
Energy and vision
,”
Philos. Mag.
27
,
1
23
(
1889
).
41.
P.
Cinzano
,
F.
Falchi
, and
C. D.
Elvidge
, “
Naked star visibility and limiting magnitude mapped from DSMP-OLS satellite data
,”
Mon. Not. R. Astron. Soc.
323
(
1
),
34
46
(
2001
).
42.
Bradley E.
Schaeffer
Telescopic limiting magnitudes
,”
Astr. Soc. P.
102
,
212
229
(
1990
).
43.
H.
Richard Blackwell
Contrast thresholds of the human eye
,”
J. Opt. Soc. Am.
36
,
624
643
(
1946
).
44.
Andrew
Crumey
, “
Human contrast threshold and astronomical visibility
,”
Mon. Not. R. Astron. Soc.
442
(
3
),
2600
2619
(
2014
); e-print arXiv:1405.4209 [astro-ph.IM].
45.
S.
Hecht
,
S.
Shlaer
, and
M. H.
Pirenne
, “
Energy at the threshold of vision
”,
Science
93
(
2425
),
585
587
(
1941
).
46.
S. I.
Vavilov
, “
Eksperimentalnye issledovaniya svetovyh kvantovyx fluktuacij visualnym metodom
,”
Usp. Fizi. Nauk
36
(
3
),
247
283
(
1948
) (in Russian).
47.
S. I.
Vavilov
,
The Microstructure of Light (Investigation and Essays)
(
USSR Acad. Sci.
,
Moscow
, 1950) (in Russian); German translation: Berlin, Akad. Verl. VII (
1953
).
48.
Gislin
Dagnelie
 et al,
Visual Prosthetics: Physiology, Bioengineering, Rehabilitation
(
Springer
,
New York
,
2011
), p.
398
.
49.
Y.
Sheffer
, “
The Big Bang Was Not That Bright
,” e-print arXiv:1803.10771 [physics.pop-ph].
50.
Planck Collaboration
, “
Planck 2015 Result. XIII. Cosmological parameters
,”
Astron. Astrophys.
594
,
A13
(
2016
).
AAPT members receive access to the American Journal of Physics and The Physics Teacher as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.