Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Hubble Constant and the Expansion Age of the Universe   The Hubble constant, which measures the expansion rate, together with the
total energy density of the Universe, sets the size of the observable Universe,
its age, and its radius of curvature. Excellent progress has been made recently
toward the measurement of the Hubble constant: a number of different methods
for measuring distances have been developed and refined, and a primary project
of the Hubble Space Telescope has been the accurate calibration of this
difficult-to-measure parameter. The recent progress in these measurements is
summarized, and areas where further work is needed are discussed. Currently,
for a wide range of possible cosmological models, the Universe appears to have
a kinematic age less than about 14 +/- 2 billion years. Combined with current
estimates of stellar ages, the results favor a low-matter-density universe.
They are consistent with either an open universe, or a flat universe with a
non-zero value of the cosmological constant.

23
The categories of this paper are: solv-int
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
33
The categories of this paper are: plasm-ph
0
The categories of this paper are: hep-ph
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga
25
The categories of this paper are: patt-sol
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph