The Sciences

The Universe is expanding at 74.2 km/sec/Mpc

Bad AstronomyBy Phil PlaitMay 7, 2009 8:11 PM


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How far away are things? That is perhaps the most basic question in astronomy, and in some ways the most aggravating. For nearby stuff -- and by that I mean everything from the Moon out to stars about 1000 light years away -- we can measure distances directly. Bouncing radar pulses off of planets gives us their distance, and in some cases we've sent probes to them so their distances are extremely well known. For nearby stars we can use parallax, which is using the motion of the Earth around the Sun to see how that affects how we see the position of the stars in the sky. But for distant galaxies, getting their 10-20 is a lot harder. That's why objects like the spiral galaxy NGC 3021 are so useful:

OOoo. Ahhhhhh. That pretty li'l thing is a cosmic bootstrap device. It and other galaxies like it have been under the scrutiny of my old friend Adam Riess, who has some big questions for them. Like, how fast is the Universe expanding? And NGC 3021 may just have the answer. That particular galaxy has two extremely useful properties. One is that it contains a type of variable star called a Cepheid variable, which are stars that literally pulsate in size and brightness. The time it takes to change in brightness is directly related to its absolute brightness: the more luminous the star is, the longer it takes to pulse. If you measure its precise pulse period, you find the star's true brightness. Compare that to how bright it appears in a telescope, and voila! You get the distance.

Hubble's view of NGC 3021. Click to get to much more cromulently embiggened images.Both images credit: NASA, ESA, and A. Riess (STScI/JHU)

Cepheid measurements yield pretty good distances for nearby galaxies, but after a certain distance they are too faint to see. We need a better rung on this distance ladder... and we have one. Supernovae. Type I supernovae are a kind of exploding star that we think (well, we're actually pretty sure) all explode in such a way that their absolute brightness can be determined, so, like Cepheids, their true distance can be found. And we can see them out for hundreds of millions of light years, which is really really far away. This makes them incredibly powerful beacons for astronomers. The cool thing is, NGC 3021 and others like it have Cepheids in them, and are also known to have hosted Type 1 supernovae! Over ten years ago, a Type I went off in NGC 3021, and very precise measurements of it were made, including how far away it was. So for galaxies like NGC 3021 we have two methods of measuring distances which can be tied together in a single galaxy that can be observed with a single telescope, like Hubble. That means that uncertainties in the distance measurements using the two systems can be hammered away, and we get more reliable measurements. And since we can see supernovae out to such fantastic distances, that means we can accurately measure the expansion of the Universe. Using supernovae to measure the distances of remote galaxies can be compared to the distances we get for those using the redshift, the Doppler-like shift in the starlight coming from those galaxies. So again, we're tying together different ways of measuring distances, allowing us to refine just how this old cosmos of ours is, and how quickly it's expanding. Adam Riess and his team observed quite a few galaxies in this way, and figured just how fast the Universe is growing to unprecedented accuracy. His result: 74.2 ±3.6 kilometers/second/megaparsec. That means for every megaparsec (about 3 million light years) you go out, the Universe is expanding 74.2 km/sec faster. So a galaxy 10 Mpc away would be moving away from us at 742 km/sec. Adam's measurement jibes well with other measurements, so there is reason to be confident in his results. By knowing this number accurately, all we have to do is measure how fast the galaxy is moving away from us -- a very easy measurement to make -- and we can find its distance. Of course, it's more complicated than that, but that's the basic idea. But by nailing down all these numbers, we can in turn nail down such things as how much dark energy is in the Universe, and maybe even rule out some theories as to what this mysterious stuff is. It's pushing on the fabric of space and time, making the Universe swell faster every second of every day, and we have no clue what it really is. Well, that's unfair: we have lots of clues, but we don't know what's causing it. Observations of NGC 3021 and other galaxies like it will help us unravel some of these mysteries, which are among the biggest in science today. Who would have thought that all this could happen just by figuring out how bright some stars are? Oh yeah, scientists did. That's what they do.

Cepheids in NGC 3021.

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