• Question: I understand cosmic waves originated as gamma waves shortly after the big bang but having stretched out as the universe expanded have now become microwaves. If the Universe is continuing to expand, do we expect the wavelengths to continue to increase and are there any implications for this?

    Asked by Robert to Susan, Scott, Savannah, Philippe, Meirin, Lucy, Jose, Harrison, David, Carsten, Anne on 7 Nov 2019.
    • Photo: Susan Cartwright

      Susan Cartwright answered on 7 Nov 2019:


      I assume by “cosmic waves” you mean the cosmic microwave background (CMB). It is a bit misleading to say that this originated as gamma rays: in the early universe, photons and charged particles interacted so readily that the universe was completely opaque to light. The CMB photons that we observe today originate from the time at which the universe first became transparent to light, which happened when the free protons and electrons first combined to form neutral hydrogen (cosmologists conventionally call this “recombination”, but that’s misleading, because they’d never been combined before). Photons interact with neutral atoms much less than they do with charged particles, so when this happens it becomes possible for photons to travel freely for very long distances. At this point, the photons are definitely not gamma rays: the temperature is about 3000 K, so most of them are infra-red. Their wavlengths have expanded by about a factor of 1100 since then, so their temperature is now 2.7 K.

      As the universe expands, the temperature of the CMB will continue to decrease, but bear in mind that it’s taken it 13.8 billion years or so to decrease from 3000 K to 2.7 K, so don’t expect any observable change in your lifetime! We can, however, observe how the gas in distant clusters of galaxies interacted with the CMB billions of years ago, and from that we can deduce that the CMB was, as we would expect, hotter in the distant past.

      So, in summary, the answers to your questions are yes and no respectively!

    • Photo: Scott Lawrie

      Scott Lawrie answered on 8 Nov 2019:


      An interesting implication of the cosmic microwave background radiation (CMBR) is on black hole evaporation. An isolated black hole will eventually shrink and disappear by emitting Hawking radiation. However no black hole is ever truly isolated – even if there is no dust, gas, stars or people to eat nearby! This is because the ever-present CMBR is always hitting the black holes. Although it’s weak, the amount of energy added to a black hole by the CMBR is still larger than the amount of energy lost through Hawking radiation, so even isolated black holes will grow slowly by eating the CMBR. However one day, once the CMBR wavelength has been stretched further, losing its energy, there will truly be nothing for black holes to eat and they can evaporate away. Note that this is a mind-bogglingly large number of trillions of years into the future 🙂

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