Missing Matter in Universe Found | Hacker News Reader

Missing Matter in Universe Found

caltech.edu

31 pointsfrankish22 comments

> The vast majority of matter in the universe is dark—it is entirely invisible and detected only through its gravitational effects

They state like dark matter is a fact. Isn't it a hypothesis?

> The FRBs shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it's too faint to see

Light slows down??

1. It is a phenomenon not a hypothesis. Dark matter is a collection of observational facts that indicate an unknown source of gravity. 2. Yes, in any medium lights slows down. This is what refractive index measures.

Light is always light and has always the same speed but its path in a gas is less straightforward than in a vacuum because of the interactions with atoms. It takes longer to get through. Its speed as we can measure it is c divided by the refractive index of the gas, if I'm not wrong.

Same thing for light in a liquid or in a solid. Example: speeds of radio waves in networking cables https://en.wikipedia.org/wiki/Velocity_factor#Typical_veloci...

Yep. And then journalists wonder why people don't like them. How much harder would it be to write "measuring how much longer would it take for the light to get to us" without making people feel gaslit next time they are told speed of light is obviously always the same

This was written on a college website and probably assumes an audience that is familiar with basic physics, since refraction is something we learn in grade school.

Also, who doesn't like journalists? I appreciate journalists a lot, even when they make mistakes. They provide a valuable service.

Is assuming the audience has knowledge of fundamentals or the intellectual curiosity of their own to pursue it wrong? If every article had to describe the most basic aspects of physics, they would be books and significantly harder to understand due to breathiness.

If it was written for scientific audiences it would use proper terms instead of factually wrong "light slows down". It's not "easier to understand" when it is wrong. Speed of light does not change.

The speed of light in a vacuum does not change. The speed of light in a non-vacuum medium can be different than the speed of light in a vacuum, however. And light passing from one medium to another changes speed (and is refracted). See https://en.wikipedia.org/wiki/Refractive_index

Weird, those of us who knew what refraction was knew what they meant just fine. Saying light "slows down" in alternative mediums is the normal way to communicate it.

Earlier: https://news.ycombinator.com/item?id=44302414

Always in the last place you look.......

gods I've always hated that saying.

Well, duh...

Does this remove the need for WIMPs and other exotic explanations for dark matter?

This study accounts for missing ordinary matter, not dark matter. The linked article makes this clear in the first paragraph. Sometimes I wonder if the first commenters (and often top commenters) on HN read the article at all or just respond based on the headline, because these comments often seem barely related to the actual article content.

> Sometimes I wonder if the first commenters (and often top commenters) on HN read the article at all ...

From the HN guidelines:

"Please don't comment on whether someone read an article."

I had read the entire article, and it was not clear.

> Unlike dark matter, ordinary matter emits light of various wavelengths and thus can be seen. But a large chunk of it is diffuse and spread thinly among halos that surround galaxies as well as in the vast spaces between galaxies.

> Due to its diffuse nature, roughly half of ordinary matter in the universe went unaccounted for and had been considered "missing"—until now.

Not being mean, genuine question: How would you improve the clarity of this?

This has nothing to do with dark matter, it's about the missing baryonic matter. And this result just confirms what most people thought anyway, but it's still rather important because it's a very solid result so we don't need to call it "missing matter" anymore.

No.

It was on the table under the dishcloth!

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