Ask Ethan #54: What’s the Earliest Signal from the Universe? (Synopsis)

“From earliest times, humans — explorers and thinkers — have wanted to figure out the shape of their world. Forever, the way we’ve done that is through storytelling. It is difficult to let the truth get in the way of a good story.” -Adam Savage

When we look back into the Universe, there's a wonderful, remarkable story that it tells us about itself. The more light we gather, of different wavelengths and over longer periods of time, the more we can discover.

Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team. Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team.

But when we look out at the Universe, there's a limit to what we can see. Light, after all, needs not only a medium to travel through, but a medium that won't destroy or randomize whatever information it's carrying. And I'm sorry, everyone, but a hot, dense plasma will do exactly that.

Image credit: Martin Hendry, via Image credit: Martin Hendry, via

But does that mean the Cosmic Microwave Background is the limit to how far back we can see? Or, in principle, can we go farther? Find out -- with a bonus -- on this week's Ask Ethan!

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@Ethan: I've got two comments I'd like to leave about this article, but I'm going to do them in separate comments as they're quite disjoint.

You wrote, regarding gravitational waves, "They can be emitted but — as far as we know — not absorbed by changes in the configurations of masses." That seems wrong to me, somehow.

The principle of detailed balance assumes that any physical process (okay, any physical _micro_process) is reversible. That should be true of gravitational waves, as well. Here's a simple thought experiment: A pair of orbiting masses, which has a net quadrupole moment, will emit gravitational waves. If a pair of masses (for example a couple of close asteroids in the solar system) is "hit" by passing gravitational waves, then their motion should be affected inversely, absorbing some of the energy of the passing wave.

I'm not doing a quantitative calculation here -- the effect may well be immeasurably small by today's technology -- but it's a question of principle. You imply that gravitational waves are a purely emissive phenomenon; I think that must be technically not correct. If I'm missing something subtle, I'd sure appreciate a clarification!

By Michael Kelsey (not verified) on 19 Sep 2014 #permalink

@Ethan: Towards the end of your piece, you talk about what led you onto your path. My own inspiration was similarly non-academic, and what you wrote really resonated for me.

I think it is very interesting, and instructive, that the kind of non-intellectual, "visionary" or "revelatory" experience, so touted by the religion-mystics, can lead someone into a passionate career in real science.

When I was young, I went through a pretty Sundays-only Catholic upbringing, including catechism studies and their sequelae. At the same time, I was quite thoroughly immersed in science and science fiction (old-school authors like Asimov, Bova, Campbell and Clarke). I realized that science could do all of the things "faith" was supposed to do, but I could actually be a part of it!

Why are we here? Let's do some investigation and find out! Where did the stars come from? Let's study that, and come up with a real answer, instead of taking some dead author's word for it. I don't ever get to actually _answer_ those questions myself, but I get to contribute my small load of bricks to the ever growing structure.

By Michael Kelsey (not verified) on 19 Sep 2014 #permalink

I noticed this: "Gravitational waves (or gravitons, if you prefer a particle-based description) are ripples in the very fabric of space itself. They move at c, the speed of light through a vacuum, but all they do is distort space".

Sorry to be picky, but gravitational waves are ripples in spacetime rather than space. Space isn't distorted or curved where a gravitational field is, see Baez. Our plot of measurements is curved and we say spacetime is curved, but space is inhomogeneous, see . Then see The Role of Potentials in Electromagnetism by Percy Hammond and look at the sentence near the end-note: “We conclude that the field describes the curvature that characterizes the electromagnetic interaction.” Electromagnetic waves are ripples in space.

By John Duffield (not verified) on 20 Sep 2014 #permalink

Cosmic background neutrinos would presumably give yet another window on the early moments of the Big Bang. Gravitational waves probe the inflationary era - what era would neutrinos probe?

By Robert Oerter (not verified) on 22 Sep 2014 #permalink

@Robert Oerter #4: Unfortunately, the relic neutrinos don't probe back much farther than the CMB. Essentially, the Universe becomes transparent to them when the temperature drops below of order the Z^0 mass (say several tens of GeV).

At that point, the cross-sections for v p -> n e+ and v n -> p e- become small (why we call the interaction "weak" at our eV to MeV energies), and the neutrinos start to free-stream.

For comparison with the CMB, the relic neutrino temperature is computed to be about 1.95 K (so a bit less than a degree colder), reflecting the additional time they've been stretched by the expansion.

That corresponds to a neutrino energy of micro-electron-volts, which is entirely undetectable (the cross-section is too low even for our most massive detectors to ever see a distinguishable event).

By Michael Kelsey (not verified) on 22 Sep 2014 #permalink