Measuring the Hubble Constant Better 102
eldavojohn writes "The Hubble Constant is used for many things in astrophysics: from determining how fast things are moving away from us, to the total volume of the universe, to predicting how our universe will end. The current best value for the Hubble Constant is 74.2 ± 3.6 (km/s)/Mpc according to recent conventional methods and the recently restored Hubble Telescope. Most astronomers agree that that's within 10% of its actual value. Researchers now claim that they might be able to get to 3% using water molecules in galactic disks to act as masers that amplify radio waves, to analyze galaxies seven times as far away as the current measurements. The further away the 'standard candle' is, the more assured they can be that local effects are not skewing the measurements. From one of the researchers: 'We measured a direct, geometric distance to the galaxy, independent of the complications and assumptions inherent in other techniques. The measurement highlights a valuable method that can be used to determine the local expansion rate of the universe, which is essential in our quest to find the nature of dark energy.' Once the Square Kilometer Array is completed, they hope to get even closer to the actual value."
Re:Hubble constant now a misnomer (Score:1, Informative)
Doesn't this constant place an additional limit on the size of the universe (or at least the part of the universe we're ever going to see) ?
c / 74.2 km/s * Mpc = 300000 / 74.2 * 3 261 636.26 lightyear (1 Mpc = 3 261 636.26) or about 1.31872075 Ã-- 10^10 lightyear, about 13 billion lightyear.
Because at that distance, the stars would be moving away from us at light speed, so in reality there's an event horizon between us and stars at that distance. Light from stars further away would never reach us, due to it having unlimited redshift.
As you can see, if the hubble constant becomes bigger, the universe shrinks. If it lowers, the universe becomes bigger.
Great Experimental Idea (Score:5, Informative)
1 - Distance measurements are currently kludged together very carefully using bridging. We use one measurement, for instance parallax based on the Earth's movement over 6 months, to show us the distance to a star that has some particular properties and which our models say should always be a certain luminosity. The parallax measurement has error bars.
2- Then we find a much more distant star of that same type that is near a particular type of supernova, and measure its brightness, comparing that to the brightness of our first star to give the distance to the distant star, and thus the supernova as well. That has bigger error bars.
3- Then we look for that type of supernova in very very distant galaxies. Supernovae are brighter than the rest of their galaxy put together while they're burning hot, so we can see them at tremendous distances. We use the measured brightness of that supernova to determine the distance to its galaxy.
4- Then we pair the knowledge of its distance with its velocity with respect to us, which we can determine through redshifting of something with a familiar spectrum. More error bars. That becomes a single point for the determination of the Hubble Constant (and yes, the "constant" is changing).
With only a cursory glance at TFA, it looks to me like this is a way to skip to step 3 or 4, thereby avoiding the need to bridge these length-scales using several techniques.
Re:Hubble constant now a misnomer (Score:1, Informative)
And how come it's measured in some stupid space unit? It's a frequency so it wants hertz!
The Hubble constant tells you the speed that astronomic objects move away from us (or from any point in the universe, cf. Galilei invariance) depending on how far it already is, hence (km/s)/Mpc. An object at the distance of 1 Mpc moves away at approximately 74 km/s.
Now what exactly does the value in Hz tell you? Nothing.
Re:Great Experimental Idea (Score:3, Informative)
More details here: http://en.wikipedia.org/wiki/Cosmic_distance_ladder [wikipedia.org]
Re:Volume of universe? (Score:3, Informative)
Re:Good Enough? (Score:3, Informative)
> When I was doing university physics with a slide rule, three significant figures ( 74.2
> ± 3.6 (km/s)/Mpc) was good enough for anything.
When I was doing university chemistry with a book of log tables four significant figures was barely good enough for my homework.
Re:Volume of universe? (Score:5, Informative)
The observable universe [wikipedia.org] is the region of space we can see. The universe has a finite age, so there is a finite distance over which we can see. Any further than that, and light literally hasn't had enough time to reach us. So there is indeed a boundary beyond which we cannot observe. This boundary recedes as time goes on. The universe is ~13.5 billion years old, but because the universe was expanding during all that time, the observable universe is bigger than just 13.5 billion light-years (see comoving distance [wikipedia.org])... in fact it is 46.5 billion light-years in radius.
Now there is every indication that the universe extends beyond the cosmological horizon. So as the universe ages, we see more and more of the full universe, which is much larger than our observation volume. So how big is the universe as a whole? Our best understanding right now is based on the curvature of spacetime [wikipedia.org]. If spacetime at large scales is curved, then the universe can loop back upon itself and thus the universe is finite. If spacetime is perfectly flat on cosmological scales, then in fact the universe as a whole is infinite in size.
Our best measurements indicate the universe is flat, within error. Our best theories of the origin of the universe, coupled with available data, generically predict that the universe is infinite. So our current best answer is that the universe is infinite in size/volume. A strange result, perhaps, but that's our best understanding of the current data. Now there are indeed errors on our measurements, so our universe could be smaller. But the curvature is so small that it implies our universe contains at least [mit.edu] 1000 Hubble volumes [wikipedia.org] (the Hubble volume is the surrounding space beyond which nothing is accessible since matter is receding faster than light). Others have analyzed the night-sky looking for 'repeat patterns' that would be expected for smaller closed universes, and no such patterns have been found.
So the observable universe is finite (but ever-expanding), and the full universe is considerably larger (infinite according to our current best data and theories).