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The Question

(Submitted March 21, 1997)

I read, with interest, the recent reports of the Hipparcos Satellite results. The revised data for stellar distances gives an age of the oldest stars to be about 11 billion years, less than the 15 billion originally given. This partly solves the paradox of the oldest stars being older than the age of the Universe.

However, how does the new stellar distance data affect the value of the Hubble constant (if at all) and how will this alter the estimated age of the Universe. Also will the revised data have any consequence for the value of omega,the critical mass of the Universe.

The Answer

Since I'm not an expert on this subject, I have asked a colleague, Dr. Richard Mushotzky, for input. Here is a brief summary.

There are two Hipparcos results with cosmological implications.

(1) They measured trigonometric parallaxes of main sequence stars with similar chemical compositions and ages to those of Globular Cluster (GC) main sequence stars. With this, the distances to GCs have been revised upwards, therefore GC stars are intrinsically brighter than previously thought, and therefore younger. (The standard tool for measuring the ages of GCs has been the main sequence turnoff --- which is now estimated to be occurring at a brighter absolute magnitude.)

This has no direct implications for H0 (the Hubble constant); see below for the implications on Omega.

(2) They have also measured trigonometric parallaxes of several nearby Cepheids. Most Cepheids are at or beyond the limits of the capabilities of Hipparcos, and there are some complications, but it appears that the Cepheids are brighter than previously thought. This would reduce the estimates of H0 based on Cepheids.

However, this may not be as important as you might think, because there are now a couple of rapidly improving methods of measuring the value of H0 that are completely independent of the Cepheid distance scale (Sunyaev- Zeldovich effect and time delays in Gravitationally lensed systems). They are all converging to about 60-65 km/s/Mpc (the best Cepheid based values tended to be around 70).

One can derive a constraint on Omega by requiring that the Globular clusters be younger than the Universe (you can also play with the "cosmological constant" here, but let's ignore this possibility for the moment). This is because one needs the value of Omega (which is related to the deceleration of the expansion of the Universe) as well as that of the Hubble constant to calculate the age of the Universe. With this method, the Hipparcos discovery on the GC age raises the allowed value of Omega somewhat, to be roughly consistent with the observationally derived values which tend to be around 0.3.

Best wishes

- Koji Mukai
for Imagine the Universe!
with a big help from Dr. Mushotzky

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