Arguably the most important Cosmological discovery ever made is that ourUniverse is expanding. Its stands, along with the Copernican Principle--- that there is no preferred place in the Universe, and Olbers' paradox--- that the sky is dark at night, as one of the cornerstones of moderncosmology. It forced cosmologists to dynamic models of the Universe,and also implies the existence of a timescale or age for the Universe.It was made possible by in part by Vesto Slipher's measurements of theapparent radial velocities of nebulae, but primarily by Edwin Hubble'sestimates of distances to nearby galaxies. Hubble deservesthe credit for the discovery of the expansion, even though papersby Georges Lemaitre and H. P. Robertson using Hubble's data on thevelocity-distance relation preceeded his 1929 landmark, because it was hissystematic program of measuring galaxy distances and his 1924 discovery ofCepheid variable stars in M31 and his actual plot of the relation thatfinally convinced the community at large.
Low level controversy ensued almost immediately. Hubble's initialvalue for the expansion rate, now called the Hubble Constant, was approximately500 km/s/Mpc or about 160 km/sec per million-light-years. The expansion age of theUniverse inferred from this was only 2 Gyr, but by the 1930's, radioactivedating of rocks had already shown geologists that the age of the Earth was3 Gyr. Astronomically Hubble's value also caused a bit of trouble, becausethe scale of the Milky Way itself was moderately well established and Hubble'scalibration implied that the Milky Way was far larger than any other nearbygalaxy except possibly Andromeda. The astronomer Jan Oort took Hubble'sscale to task for this reason in a 1932 paper, but the astronomicalcommunity continued to support and use Hubble's value. The solution, at least to theEarth versus the Universe problem, came in the 1950's from a combinationof effects including Walter Baade's discovery of Population II starsand his subsequent recalibration of the period-luminosity relation forpopulation I Cepheid variables followed by the realization of a confusionproblem at large distances. What Hubble had thought were individualstars in his most distant galaxies were actually star clusters, thus he hadnot been observing ``standardcandles,'' objects whose absolute luminosity did not vary with distance.
However, all was not settled. In the 1960's a great controvesy over thevalue of the expansion rate grew again. Allan Sandage, Hubble's successorat the Mt. Wilson and Palomar Observatories, continued to drive down thevalue of H0. In theclassic paper by Humason, Mayall and Sandage (1956), the value determined was180 km/s/Mpc. Two years later, in 1958 Sandage pubished a value of75 km/s/Mpc, and by the early 1970's estimates from Sandage and his longtimecollaborator Gustav Tammann were hovering around 55 km/s/Mpc. The dramaticchange over 5 decades is shown in figure 2. Meanwhilethe competition, in the form of Sidney vandenBerg and Gerard deVaucouleurscontinued to obtain values near 100 km/s/Mpc. By the late 1970's, thisbimodality remained in the estimates of H0 and the middle ground waslittered with the bruised and battered remains of young astronomers attemptingto resolve the dispute between the two sides.
The resolution of the problem came from the telescope that bears Hubble'sname. In the early 1980's, prompted by the director of the Space TelescopeScience Institute, Riccardo Giacconi, NASA and StScI convened four panelsto discuss the concept of Key Projects. These wereenvisioned as large observational programsof such significant scientific impact that blocks of Hubble Space Telescopetime would be set aside and separately proposed for to ensure that theseprojects would be completed in the early years of the HST missionin case the telescope failed after only a few years. The panelsidentified three such projects, a study of the nearby intergalactic mediumusing quasar absoprtion lines, a medium deep survey to be composedof exposures taken in parallel (basically turning on the cameraswhenever one of the other instruments was primary), and a projectto determine the Hubble Constant. The following summer, a number of astronomers interested in the Cosmic Distance Scale met at the Aspen Center for Physics in 1985 to discuss what to do next.Ostensibly, the program was arranged to give the local theorists(Aspen being the summer home of a large number of theorists!)the current view on H0, but the unstated reason for the meetingwas to form a team or teams to propose for HST time. The grouptried to combine to write a single proposal, but there reallywas no way to get the old timers to work with the young turks.In the end, a team of thirteen astronomers agreed to continuemeeting to plan the HST project. The first draft of the proposalwas prepared in Tucson, under the leadership of Marc Aaronson,a few months later. The original team members included Marc, JeremyMould, Rob Kennicutt, Wendy Freedman, Sandy Faber, Holland Ford,Jim Gunn, John Hoessel, Garth Illingworth, John Graham, Peter Stetson,Barry Madore and myself.
The primary goal of the HST Key Project on the Extragalactic Distance Scale,as the program came to be called, was to beat down the errors, bothexternal and internal, in the calibration of the distance scale toderive a value of the Hubble Constant good to ten percent. Examination ofthe ``error trees'' for almost all previous determinations of H0showed that the nearly factor of two range in derived values wasnot unexpected given the large number of contributing parameters. Ateach rung of the distance ladder subtle and sometimes not-so-subtlechoices introduced both larger and larger discrepancies and errors.The plan we made is shown in the flowchart below.
The H0 Key Project has reached its goal of the determination of theexpansion rate of the Universe and the Cosmic Distance scale to10%. Links to the work on individual galaxies can be found at the HST H0 Key Project website. An example of one of our Cepheid fields, that for the Fornax Cluster galaxy NGC1365 is shown here.
This image shows the field of the Wide-Field Planetary Camera 2 superimposedon a groundbased image of the galaxy taken with the Dupont telescopeat Las Campanas (see Silberman et al 1999, ApJ 515, 1).
For fun, the plots below show the time evolution of ourknoweldge of the Hubble Constant, the scaling between radial velocityand distance in kilometers per second per Megaparsec, since it was first determined by Lemaitre, Robertson and Hubble in the late 1920's.
Note here that the first point is actually from a paper by G. Lemaitrein 1927 based on distances to galaxies derived and published by Hubble.The second is from H. Robertson, also based primarily on Hubble's data. Hubble himself finally weighed in in 1929 at 500 km/s/Mpc. Also, very early on, the Dutch astronomer, Jan Oort, thought something was wrong with Hubble's scale and published a value of 290 km/s/Mpc, but this was largely forgotten.
The first major revision to Hubble's value was made in the 1950's due to the discovery of Population II stars by W. Baade. That was followed by other corrections for confusion, etc. that prettymuch dropped the accepted value down to around 100 km/s/Mpc by the early 1960's.This figure is also available in postscript format at hplot.ps
This plot shows modern (post HST) determinations, including resultsfrom gravitational lensing and applications of the Sunyaev-Zeldovicheffect. Note the very recent convergence to values near 65 +/- 10 km/sec/Mpc(about 13 miles per second per million light-years). The data for this plot isat hubble.plot.dat and will be updatedperiodically as part of the HST Key Project on the Extragalactic DistanceScale. Currently, the old factor of two discrepancy in the determinationof the cosmic distance scale has been reduced to a dispersion of the orderof 10 km/s out of 65-70, or 15-20%. Quite an improvement! The summaryresults from the HST H0 Key project are plotted below. With some slightmodifications to the Cepheid scale zeropoint, we believe our best valuefor the local H0 determination is around 71 (+/- 7) km/s/Mpc.
The flip side of this is the still sad state of affairs governing the absolutecalibration of the Cepheid scale. A both serious and humorous reviewwritten from a historical perspective by Nick Allen can be found at
Its definitely worth a gander. The uncertainties in the local determination ofthe Hubble Constant are still dominated by the uncertainty in the CepheidP-L calibration, followed by uncertanties in the local flow field (non-Hubbleexpansion galaxy velocities). The current state of published measurementsis seen below.
One major additional change in the debate since the end of the 20th century has been the discovery ofthe accelerating universe (cf. Perlmutter et al. 1998 and Riess et al. 1998)and the development of "Concordance" Cosmology. In the early 1990's,one of the strongest arguments for a low (~50 km/s/Mpc) value of the HubbleConstant was the need to derive an expansion age of the universe that was olderthan, now, the oldest stars, those found in globular star clusters. The bestGC ages in 1990 were in the range 16-18 Gyr. The expansion age of theUniverse depends primarily on the Hubble constant but also on the value of variousother cosmological parameters, most notably then the mean mass density over theclosure density, &OmegaM. For an "empty" universe, the age isjust 1/H0 or 9.7 Gyr for H0 = 100 km/s/Mpc and19.4 Gyr for 50 km/s/Mpc. For a universe with &OmegaM = 1.000,the theorist's favorite because that is what is predicted by inflation, the age is 2/3 of that for the empty universe. Soif the Hubble Constant was 70 km/s/Mpc, the age of an empty universe was 13.5Gyr, less than the GC ages, and if &OmegaM was 1.000 as favored bythe theorists, the expansion age would only be 9 Gyr, much much less than theGC ages. Conversely if H0 was 50 km/s/Mpc, and &OmegaMwas the observers' favorite value of 0.25, the age came out just about right.Note that this still ruled out &OmegaM = 1.000 though, inspiringat least one theorist to proclaim that H0 must be 35! The discovery of acceleration enabled the removal of much of this major remaining discrepancy in timescales, that between the expansion age of the Universe and theages of the oldest stars, those in globular clusters. The introductionof a Cosmological constant, &Lambda, one of the most probable causes for acceleration, changes the computation of the Universe's expansion age. A positive &Omega&Lambda increases the age. The Concordance model has an H0 = 72 km/s/Mpc,an &OmegaTotal = 1.0000..... made up of &Omega&Lambda=0.73 and &OmegaMatter=0.27. Thosevalues yield an age for the Universe of ~ 13.7 Gyr. This alone would nothave solved the timescale problem, but a revision of the subdwarf distance scale based on significantly improved paralaxes to nearby subdwardsfrom the ESA Hiparcos mission, increased the distances to galacticglobular clusters and thus decreased their estimated ages. The mostrecent fits of observed HR diagrams to theoretical stellar models(isochrones) by the Yale group (Demarque, Pinsonneault and others) indicates that the mean age of galactic globulars is more like 12.5 Gyr,comfortably smaller than the Expansion age.
This represents the sum of all the data (not all independent) availablesince the repair of the Hubble Space Telescope.
Information on these and other projects also may be gotten via links below.
Its also worth looking at Ned Wright's Cosmology Calculator if you want tocalcuate parameters based on the best available current numbers:
We have also endeavored to maintain a listing of published values of theHubble constant at
If you find errors or know of missing references, please contact the author.
Links to other information on the Hubble Constant:
John P. Huchra <huchra@cfa.harvard.edu> Copyright 2008
[8]ページ先頭