Tuesday, February 10, 2009

Dark matter

As mentioned in the summary, this blog will be my attempt at explaining what I study to my mom and any other person out there who might be interested. What do I do? I am trying to directly detect dark matter.

Today, the scientific community generally accepts that 95% of the universe is made up of stuff that we’ve never seen before and do not understand; the chart at the right shows the composition of our universe as measured by the NASA/WMAP satellite (the subject of a future series of posts). We think about a quarter of this unknown density is in the form of dark matter. Although the phrase “dark matter” sounds sci-fi and romantic, it’s actually a simple description of what we think it is – matter that does not interact in the usual way with light, unlike the ordinary matter that makes up the Earth, people, the Sun, and everything else that we can see in the sky at night (the remaining 5%).

There's a lot of circumstantial evidence for the existence of dark matter (rotation curves, gravitational lensing, the cosmic microwave background, etc.), and I hope to go through much of it in future posts. For now though, I will briefly describe the simplest piece of evidence, the rotation of galaxies.

Because of gravity, galaxies orbit about their center, just as the Earth orbits around the Sun – we’ve all probably seen nice pictures of spiral galaxies (like this picture of Andromeda), which are spirals because of the rotation. We can figure out the speed of the rotation using the Doppler effect, which says that the frequency of an observed wave will be shifted depending on the relative speed of the source and observer. A familiar example of the Doppler effect is that the frequency and pitch of a police siren will change to a listener on the sidewalk as the police car passes by (A cartoon illustration).

Since light is also a wave, the frequency and wavelength of light coming from a distant galaxy will also be shifted if the galaxy is moving – this is known as “red shift.” In a rotating galaxy, one side is spinning away from us while another side is spinning towards us, and the difference in shift from one side to the other can tell us how fast the galaxy is spinning. Simple Newtonian mechanics can predict the speed of rotation at a point in the galaxy as a function of mass and radius, and for a constant mass, the speed of rotation should decrease with increasing radius (this makes sense intuitively – the force of gravity decreases with increasing distance, so if the mass is held fixed but the distance increases, there just isn’t as much force to pull the galaxy around). Therefore, if all the matter in the galaxy were in the central, bright part of the galaxy (the part that interacts with light), we would expect the speed of rotation to decrease away from the central, bright part of the galaxy. In fact, the speed of rotation stays constant out to very large radii, much farther than the extent of the bright part of the galaxy. Therefore, there must be matter in the galaxy that we cannot see to pull the galaxy around – this is “dark matter.”

The dashed line is what we would expect to see if all the matter were in the bright part of the galaxy. The solid line is what we actually observe


  1. 臺北不婚獨子 指當台灣人 證據時效

    1-+cos(angle)=2sin(半角)平方 2cos(半角)平方 1-+sin(angle)=(sin角半-+cos半角)^two
    347.learnbank.com.tw/highschool/highschool_1_ m2.php

  2. f'=f (vv0加減vvs減加)factor接近取上方符號遠離取下方符號


  3. Hugh, I really like the concept here and your Mom asks such good questions - no wonder you are at Yale. As for me, I am an old wannabe scientist with physics degree from UVM (class '80). I am intrigued how this dark matter/energy thing has blown up into a big deal whereby the whole science community has only just discovered it is at the sophmore stage ( it knows that it knows not! ) I'll keep lurking, if you don't mind. I have a few crackpot ideas of my own that I need to disabuse myself of.

  4. I can't help but be offended by the title of your blog "physics for mom." This title, although maybe accurate in your life, is really quite sexist and offensive. As a Ph.D. scientist and a woman, I happen to know more than a few science-savvy moms. Why not title it "physics for dad"?

  5. To Allen,
    Thanks for reading, I'm glad you like the concept. I appreciate the comments on other posts so keep them coming, and hopefully we don't disabuse you of too many crackpot ideas as otherwise there's no real point...

    To anonymous,
    The title was indeed accurate to my life as my mother is the person I think of when I'm writing, and I am sorry if you are offended by it. I added a possessive pronoun in the title, which I hope helps the situation.


  6. Hi, Hugh, I'm working on an analogue of a masters thesis (here its called Diploma thesis) on CMB anisotropies and I am really intrigued by your ability of explaining those things, as I myself have often sought explanations of what I do that would be useful to non-physicists. You are really sucessful in giving a simple and yet beautiful explanation!

  7. Hello! I just discovered you accidentally. I love your description here, it's elegant, short, and easy to understand.

    I also, (as a woman with a science-savvy mom) wanted to let you know that I don't find you're title offensive or sexist. This is *your* personal blog, and anyone who is offended by the blog title (even without "my") is just passing through and not actually reading your posts. Because it is true to your life, and this is your personal writing space the title isn't sexist at all; it was title for your mom. Which, I think, is very endearing. (Most grad students don't give a darn if their moms understand what they do or not.)

  8. Great Blog!!

    In February 2010 I was at the Ninth UCLA symposium on Dark Matter for three days http://www.physics.ucla.edu/hep/dm10/index.html which was totally wonderful.

    The interest and excitement around DM and DE may well attract the attention of people without an extensive math/physics background, inspire them to get that background, and your blog is just the sort of thing to bring us up to speed.

  9. It could be that the dark matter and dark energy do not exist and they are like the aether. That is these two concepts have been been invented to keep old theory that is not working too well still valid. A theory should be falsifiable. If something does not exist, it is hard to prove it does not exist by endless searches for its existence.

    Maybe it is time to be skeptical and consider the possibility that something is seriously wrong with this 300-year-old-idea that mass mediates the gravitational force. We all know that there is a high correlation between the mass of a star and the amount of luminosity it puts out. This high correlation could be an artifact that tricks us into believing that mass mediates the gravitational force when all along it is the luminosity that is mediating it. We all know about the misleading artifact stemming from the earth rotating on it axis made the Geocentrists believe for 1500 years that all object rotates around the earth every 24 hours.
    I have performed five experiments which show that luminosity is indeed attractive. For these experiments and heat-based gravity theory go to
    http://vixra.org/abs/0907.0018 .

    It is time we wake up and get out of the rabbit hole.