Scientists further refine how quickly the universe is expanding | Hacker News Reader

Scientists further refine how quickly the universe is expanding

phys.org

23 pointsdnetesn7 comments

The paper is https://arxiv.org/pdf/1903.12097.pdf for anyone interested.

The results aren't actually that interesting. The posterior on H0 (the rate that the universe is expanding) is not at all competitive with the current best methods. They get 67 +/- 6 whereas things like Planck have error bars on the order of 1. Checkout their figure 5 to see the different error bars.

That is not to say the paper is bad at all! The method they use is pretty cool and not common (at least I haven't seen it used before, though they do mention there have been people looking at this for a while). Also, as the main source of concern for cosmologists is systematic errors, having multiple ways to measure things is very important. They call this out in their intro, "As discussed by Suyu et al.(2012), multiple paths to independent determinations of the Hubble constant are needed in order to assess and control systematic uncertainties"

I'm not sure this is a great general interest article as AFAICT there are no big picture take aways from the paper. Again, this is not to say that it is not a good paper - I found it pretty interesting! - but the cool parts are that this is a good measurement of H0 using a less common technique.

One weird thing about the article is

> In a paper published Friday, Nov. 8, in The Astrophysical Journal, Clemson scientists Marco Ajello, Abhishek Desai, Lea Marcotulli and Dieter Hartmann have collaborated with six other scientists around the world

I see the article was written by someone at Clemson, but it is a bit odd in a non-Clemson publication to explicitly call out a bunch of secondary authors, and not mention the primary author until much further down. In general in astrophysics, the vast majority of the work (except in very large collaboration papers) is done by the first author.

"What we know is that gamma-ray photons from extragalactic sources travel in the universe toward Earth, where they can be absorbed by interacting with the photons from starlight," Ajello said. "The rate of interaction depends on the length that they travel in the universe. And the length that they travel depends on expansion. If the expansion is low, they travel a small distance. If the expansion is large, they travel a very large distance. So the amount of absorption that we measured depended very strongly on the value of the Hubble Constant. What we did was turn this around and use it to constrain the expansion rate of the universe."

Does that quote say photons absorb photons? I haven't heard about something like this before.

Because they are electrically neutral, there is no direct interaction between photons, which is good, because it allows us to see each other even when there is light coming from all directions. At low energies, that's the end of the story as the interactions via intermediate particles are incredibly unlikely.

However, think about how an electron and a positron can annihilate into two photons. As the electromagnetic interaction is symmetric under time reversal, the reverse reaction should be possible, as long as sufficient energy is available. So an extremely energetic photon can combine with a photon from the background starlight and turn into an electron-positron pair, essentially being absorbed.

https://en.wikipedia.org/wiki/Two-photon_physics

Just wanted to mention that there are indications that universal constants e.g. fine structure constant, gravitational constant etc. are time varying [0].

[0] https://en.wikipedia.org/wiki/Time-variation_of_fundamental_...

Sure thing...

http://cosmology.info/database.html

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