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Measuring the Hubble Constant Better
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."
From what I know, it's been discovered in the past decade or so to not be a constant. The expansion of the universe is accelerating. This is a minor nitpick, I know. :-)
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Yep, he's still dead.
How can something of infinite size have a volume?
Once the Square Kilometer Array is completed
The name sounds impressive, but how big will it be?
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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.
"I zero-index my hamsters" - Willtor (147206)
Presumably "Erm, around 100" isn't good enough then?
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. Is our next probe going to miss M31? Oh yeah, get off my lawn too. :-)
Some mornings it's hardly worth chewing through the restraints to get out of bed.
Maybe you should present a more credible source, if you wish to be taken seriously.
Don't blame me -- I voted for Roslin.
Universe expansion will create causal separation in the future, but not the past. It doesn't limit how far away you can see something, because you are looking at something in the past, but it does prevent you from going there. Because looking backward in time, the universe is shrinking, and you can see more and more of the universe going back. Looking forward in time, everything is getting more separated, and, for far regions of space, the rate of separation is higher than light can catch up to.
In the scenario of big rip, the acceleration of the expansion continues to increase until the future light cone of every particle is separated from the future light cone of every other particle. The past light cones still intersect.
I've been following Alexander F Mayers work on Minkowski's (Einsteins Maths Teacher) space time mathematics which Einstein, who didn't understand them, called "superfluous erudition'. Mayer derives a model for the universe that does not require the universe to be expanding, let alone accelerating expansion, does not require "Dark matter" nor "Dark energy", that makes a damn sight more cosmological sense than the "Big Bang" and fits the current observations, much, much better, with no free variables like "quintessence". He makes a prediction for the LRO mission as well. http://www.jaypritzker.org/index.html
According to the article "the astronomers determined that the galaxy UGC 3789 is 160 million light-years from Earth". This translates to 49 Mpc. According to NED, the velocity (in the Cosmic Microwave Background frame) is 3385 km/s.
Therefore this measurement of the Hubble parameter is then 3385/49 = 69 km/s/Mpc.
(Unfortunately the article does not quote an uncertainty on the 49 Mpc measurement. Because of peculiar velocities, I would estimate that there is at least a 300 km/s uncertainty on the 3385 km/s velocity. )
It's also possible that this galaxy is not totally in the Hubble Flow. In other words, it might be pulled around by other nearby galaxies/galaxy clusters. All galaxies are affected by this to some extent, but with nearby galaxies (like this one), these gravitationally-caused velocities can be significant compared to the Hubble expansion-caused velocities.
I couldn't tell if you were experimenting with poor-man's cryogenics or looking for the orange sherbet.
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.
10% of 74 is 7.4, corresponding to ± 3.7; meaning that in the very worst case, where the true value is at one end of the interval, we can only get about 10% away. What the astronomers agree on is that the estimate of the uncertainty on the measurements is something like ± 3.6. This is not as trivial a matter as it would seem - it can be quite complex to calculate and is a source of many of the more embarrasing errors in science.
The Perimeter Institute recently gave a lecture on this, by Brian Schmidt, Australian National University - "The Universe From Beginning to End". I understand they will EVENTUALLY make these lectures available on their website, after they've made a bit of money by showing them on Discovery etc: https://www.perimeterinstitute.ca/en/Outreach/Public_Lectures/Public_Lectures/
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