How Do We Know Dark Matter Exists?

universetoday.com ^ | Fraser Cain

[BenLurkin]

Now, you might be saying, if we don’t know what this thing is, and we can’t detect it. How do we know it’s actually there? Isn’t it probably not there, like dragons? How do we know dark matter actually exists, when we have no idea what it actually is?

Oh, it’s there. In fact, pretty much all we know is that it does exist. Dark matter was first theorized back in the 1930s by Fritz Zwicky to account for the movement of galaxy clusters, but the modern calculations were made by Vera Rubin in the 1960s and 70s. She calculated that galaxies were spinning more quickly than they should.

... But in the last few years, astronomers have gotten better and better at detecting dark matter, purely though the effect of its gravity on the path that light takes as it crosses the Universe. As light travels through a region of dark matter, its path gets distorted by gravity. Instead of taking a straight line, the light is bent back and forth depending on how much dark matter is passes through.

And here’s the amazing part. Astronomers can then map out regions of dark matter in the sky just by looking at the distortions in the light, and then working backwards to figure out how much intervening dark matter would need to be there to cause it.

[AFPhys]

I wish that I had read your posts on this thread before I posted my #40... I would have pinged you to my thoughts if I had. Thanks for your posts.

[MrB]

These kinds of fudge factors are usually hypothesized in order to make observations fit an assumed conclusion.

The Oort Cloud comes to mind as well.

[AFPhys]

I have been somewhat disconnected from the cosmology field for about a decade, though I am very interested in it. I have looked at a couple of MOND sites discovered in search, but I am not satisfied. Can you recommend two or three for me to reconnect with this subject? (High math level is OK, and even desired.)

[stremba]

I’m certainly not an expert on the field. I have gotten most of what I know from reading science blog sites. You might try the www.science20.com website. I also find the “Starts with a Bang” blog on www.scienceblogs.com helpful, especially regarding the MOND vs Dark Matter question. (Ethan Siegel, the writer of that blog IS a cosmologist)

[AFPhys]

Thanks. I will check out those sites.

[GOPJ]

That is why current scientific understanding is that MOND is not the preferred model and that dark matter is. It’s not that scientists refused to consider that the current understanding of gravity is incorrect. Rather, it is that they did consider that possibility, but even modifying our models of gravity does not account for the observations.

Interesting...

[AFPhys]

I am realizing that it is going to take me a few weeks of study to get up to understanding the MOND vs. LCDM situation, and that by only moderately difficult math.

Div,Grad,Curl ... where are you??? I know that you’re lurking somewhere in my mind, but come out come out wherever you are!