The cosmic background is 4K. Where are the other photons? A 4K photon produced at the big bang is detected by our detector. Time stops for the photon and hence we are seeing the big bang. Shouldn't we also see the other photons from the big bang?

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    $\begingroup$ The cosmic background is 4K. No, it’s 2.725 K. $\endgroup$
    – Ghoster
    Oct 3, 2023 at 0:03
  • $\begingroup$ Where are the other photons? We detect the CMB photons that hit our CMB photon detectors. The “others” are the ones that don’t. Some of them are in your living room. $\endgroup$
    – Ghoster
    Oct 3, 2023 at 0:05
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    $\begingroup$ Photons did exist at the time of the Big Bang, but the universe was not transparent to photons until after (350+ thousand years later) the Big Bang, when the mean free path for photons was large enough to allow them to travel. See recombination. $\endgroup$
    – joseph h
    Oct 3, 2023 at 0:07
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    $\begingroup$ There is no such thing as a "4K photon." The temperature being specified (which is actually closer to 3K not 4K) is the temperature of a black body radiator. It radiates photons of all different frequencies. $\endgroup$
    – hft
    Oct 3, 2023 at 0:35
  • $\begingroup$ You seem to have a few misconceptions about photons and the Big Bang. Here are a couple videos that might help. Misconceptions About the Universe, What Actually Expands In An Expanding Universe? $\endgroup$
    – mmesser314
    Oct 3, 2023 at 0:56

1 Answer 1


The intrinsic temperature of the cosmic microwave background was much larger (in the past) than what we observe today. This is because the light emitted by the CMB has been drastically redshifted due to the expansion of the Universe over the last 13+ Giga years.

Let us use the scale factor $a(t)$ as a measure of how much the Universe has expanded between two instances of time. Also say $a(t = {\rm today}) = a_0$ Then, the temperature of the CMB at a time $t$ compared to its today's observed value is related as

$$ \frac{T(t)}{T_0} = \frac{a_0}{a(t)} = 1 + z(t)$$

where $z$ is the redshift at time $t$. Additionally, $a(t) < a_0$ (for $t < t_0$). In particular, if we go back all the way to when the Universe first became transparent, i.e., $t \approx 3 \times 10^6$ yr, then this gives $z \approx 1100$.

So we can see that the CMB would have been $\approx 1000 \times$ hotter (than today) at the era of recombination.

Note: the values quoted here are rough approximations.

  • $\begingroup$ Thanks for the explanation. Is there more evidence to determine cause and effect? Is the expansion causing the cooling or do we say that the universe is expanding because of red shift (cooling)? $\endgroup$ Oct 3, 2023 at 7:57
  • $\begingroup$ It's the expansion that is causing the redshifting of radiation to longer wavelengths. The other half of you comment makes little sense. $\endgroup$
    – S.G
    Oct 3, 2023 at 8:32
  • $\begingroup$ The other half of the comment is: Do we have any other evidence for expansion of universe other than red shifting of spectral lines? Is there any other consequence of the expansion that is measured? $\endgroup$ Oct 5, 2023 at 0:36

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