Two international teams of astronomers, using NASA's Hubble Space Telescope, are reporting major progress in converging on an accurate measurement of the Universe's rate of expansion - a value which has been debated for over half a century.
These new results yield ranges for the age of the Universe from 9-12 billion years, and 11-14 billion years, respectively. The goal of the project is to measure the Hubble Constant to ten percent accuracy.
The Hubble Space Telescope Key Project team, an international group of over 20 astronomers, is led by Wendy Freedman of Carnegie Observatories, Pasadena, CA, Robert Kennicutt, University of Arizona, Tucson, AZ, and Jeremy Mould, Mount Stromlo and Siding Springs Observatory, Australia. The group's interim results, announced at a meeting held at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, are consistent with their preliminary result, announced in 1994, of 80 kilometers per second per megaparsec (km/sec/Mpc), based on observations of a galaxy in the Virgo cluster.
"We have five different ways of measuring the Hubble Constant with HST," said Dr. Freedman. "The results are coming in between 68 and 78 km/sec/Mpc." (For example, at an expansion rate of 75 km/sec/Mpc, galaxies appear to be receding from us at a rate of 162,000 miles per hour for every 3.26 million light-years farther out we look).
Two months ago, a second team, led by Allan Sandage, also of the Carnegie Observatories, Abhijit Saha, STScI, Gustav Tammann and Lukas Labhardt, Astronomical Institute, University of Basel, Duccio Macchetto and Nino Panagia, STScI/European Space Agency, reported a slower expansion rate of 57 km/sec/Mpc.
The value of the Hubble Constant allows astronomers to calculate the expansion age of the Universe, the time elapsed since the Big Bang. Astronomers have been arguing recently whether the time since the Big Bang is consistent with the ages of the oldest stars.
The ages are calculated from combining the expansion rate with an estimate of how much matter is in space. The younger age values from each team assume the Universe is at a critical density where it contains just enough matter to expand indefinitely. The higher age estimates are calculated based on a low density of matter in space.
"A point of great interest is whether the age of the Universe arrived at this way is really older than the independently derived ages of the oldest stars," said Saha, an investigator on both Hubble teams.
"The numbers lean on the side that the stellar ages are a little lower, or that the hypothesis that we live in a critical density universe needs to be questioned," said Saha. "As further results accumulate over the next few years, we hope to tighten the constraints on these issues."
The Observations
The Key Project team is midway along in their three-year program to derive the expansion rate of the Universe based on precise distance measurements to galaxies. They have now measured Cepheid distances to a dozen galaxies, and are about halfway through their overall program.
The Key Project team also presented a preliminary estimate of the distance to the Fornax cluster of galaxies. The estimate was obtained through the detection and measurement with the Hubble Space Telescope of pulsating stars known as Cepheid variables found in the Fornax cluster. The Fornax cluster is measured to be approximately as far away as the Virgo cluster of galaxies – about 60 million light-years.
The Key Project team member who led this effort, Caltech astronomer Barry Madore said, "This cluster allows us to make independent estimates of the expansion rate of the Universe using a number of different techniques. All of these methods are now in excellent agreement. With Fornax we are now at turning point in this field."
The team is measuring Cepheid distances to the Virgo and Fornax clusters of galaxies as a complementary test. Their strategy is to compare and contrast expansion numbers from a variety of distance indicators.
The Key Project team is systematically looking into a variety of methods for measuring distances. They are using Cepheids in a large sample to tie into five or six "secondary methods." One such secondary method relates the total luminosity of a galaxy to the rate at which the galaxy is spinning, the Tully-Fisher relation. Another secondary method makes use of a special class of exploding star known as a type Ia supernova. This phase of the Hubble Constant research will be completed within another two years.
In contrast, the Sandage team focused on a single secondary distance indicator, one of the same indicators also used by the Key Project team, the type Ia supernova. Sandage maintains that these stars are "standard bombs" according to theory. He suggests that when they explode they all reach exactly the same intrinsic brightness. This would make them extremely reliable "standard candles," (objects with a well-known intrinsic brightness) visible 1,000 times farther away than Cepheids. Since they are intrinsically brighter than any other standard candle, they offer the opportunity for an accurate measurement of the Universe's overall expansion by looking out the farthest.
Although both teams are still in disagreement over the precise rate at which the Universe is expanding and on how old it is, they are optimistic that their estimates will continue to converge with further observations and analysis.
Members of the Key Project team include W. Freedman (Carnegie Observatories), R. Kennicutt (University of Arizona), J. Mould (Mount Stromlo and Siding Springs Observatories, Australia), L. Ferrarese (Johns Hopkins University), H. Ford (Johns Hopkins University), J. Graham (Department of Terrestrial Magnetism), M. Han (University of Wisconsin), P. Harding (University of Arizona), J. Hoessel (University of Wisconsin), J. Huchra (Smithsonian/Harvard University), S. Hughes (Royal Greenwich Observatory, Cambridge), G. Illingworth (University of California, Santa Cruz), B.F. Madore (IPAC/Caltech), R. Phelps (Carnegie Observatories), A. Saha (Space Telescope Science Institute), N. Silbermann (IPAC), P. Stetson (Dominion Astrophysical Observatory), and S. Sakai (IPAC).
Members of the Sandage team include A. Sandage (Carnegie Observatories), A. Saha (Space Telescope Science Institute), G.A. Tammann, and L. Labhardt (Astronomical Institute, University of Basel), F.D. Macchetto and N. Panagia (Space Telescope Science Institute/European Space Agency).
FREQUENTLY ASKED QUESTIONSWhat's the Difference Between an Open and Closed Universe?
An open universe expands forever; a closed universe expands, but decelerates until it eventually reverses direction and begins to contract; a "critical density" universe is exactly midway between these scenarios and so will expand indefinitely, always slowing down but never quite coming to a halt. If, for example, you throw an object up in the air, it falls down due to gravity. But if the object moves fast enough (say, by rocket) it can escape from the Earth. By analogy the Universe itself may not have enough density to halt its own expansion.
What's the Relationship Between Mass Density and Age of the Universe?
The rate of the Universe's expansion reflects how much gravity and hence, matter, it has. Like going up a steep hill, the galaxies outward rush should have slowed if the Universe has a lot of mass, and this implies a younger universe. If the Universe has little mass, and so is barely decelerating, then galaxies would have taken more time to reach their current positions, like rolling along a flat floor.
The rate of the Universe's expansion should be slowed by the mutual gravitational pull of all matter contained in the Universe.
Why Do Theorists Favor a Critical Density Universe?
In formulating the simplest models of the expanding universe theorists favor the notion that space contains the exact amount of matter that keeps the Universe precisely balanced between expanding forever and collapsing under gravity. Assuming such a "critical density" makes it easier to explain a number of observed properties of the space, including the large-scale structure of galaxies.
Does the Universe Contain Enough Mass To Reach Critical Density?
A fundamental problem is that telescopic observations show that the Universe contains only 1/100 the luminous (i.e., stars and galaxies) mass that it needs to reach critical density. Astrophysicists hold that dark matter must account for the rest. Observational evidence showing that dark matter affects the rotation rate of galaxies, and behavior of clusters of galaxies, boosts estimates of the amount of matter in the Universe to 10% of the value needed to reach critical density. To date the remaining 90% of the required mass to reach critical density is missing and unaccounted for.
Why Has it Taken More Than 60 Years for Astronomers to Calculate an Accurate Value for the Hubble Constant?
First, astronomers discovered that establishing an accurate distance scale to faraway galaxies has been more difficult than anticipated. Second, while astronomers can simply and accurately measure a galaxy's velocity, the measurement may not represent the expansion velocity of the Universe at that distance. The reason is that each galaxy possesses a gravitational force. Velocities are altered when more massive galaxies, which have stronger gravitational forces, pull smaller galaxies toward them.
Why Are the Teams Optimistic They Are Converging on a Single Value for the Hubble Constant?
The historically debated values of the expansion rate of the Universe have differed by up to a factor of two, but the estimates of the two Hubble teams are now within 25 percent. Hubble Space Telescope has taken this decades-old debate out of gridlock and on toward a solution. That's because Hubble can see and measure certain key celestial distance markers out to ten times farther from Earth than ground-based telescopes.
How Do the Teams Measure Cosmic Distances?
Both teams base their results on studying a class of celestial milepost marker, called Cepheid variable stars, whose pulsation rate is a direct indication of their intrinsic brightness.
Freedman's team is systematically looking into a variety of methods for measuring distances. They are using Cepheids in a large sample to tie into five or six "secondary methods." One such secondary method relates the total luminosity of a galaxy to the rate at which the galaxy is spinning, the Tully-Fisher relation. Another secondary method makes use of a special class of exploding star known as a type Ia supernova. These secondary distance indicators are needed to look deeper into the Universe to get a more representative rate for the expansion of space (the gravitational fields of nearby clusters may yield an inaccurate value because the expansion rate may be affected by the local motion of galaxies).
In contrast, the Sandage team took the "fast track" to focus on a single secondary distance indicator, one of the same indicators also used by the Key Project Team, the type Ia supernova. Sandage maintains that these stars are "standard bombs" that all reach exactly the same intrinsic brightness. They are visible 1,000 times farther away than Cepheids, allowing for an accurate measurement of the Universe's overall expansion.
Why Is Observing the Fornax Galaxy Cluster Important?
Earlier results derived from the Virgo cluster have been questioned because that cluster is so large that possible inaccuracies in the distances of individual galaxies from its center might affect some findings. The Fornax cluster is more compact than the Virgo cluster, so there is much less range for uncertainty in the distances of member galaxies from its center.
MEASURING THE EXPANSION RATE OF THE UNIVERSE
The following is a brief history of how astronomers have developed ways to measure the Universe's expansion rate.
1900 – 1910: Harvard astronomer Henrietta Leavitt begins measuring the brightnesses of stars in a class known as Cepheid variables, bright, young stars with masses of perhaps 5 to 20 times that of our own Sun. She measures the distances of stars in the Small Magellanic Cloud, a diffuse-looking nebula (from the Latin word "fuzzy"), visible in the Southern Hemisphere. Leavitt discovers that these stars reveal their intrinsic brightness by the way their light varies. This makes them reliable milepost markers for measuring astronomical distances.
1910 – 1920: Albert Einstein develops his General Theory of Relativity in 1917. Applying Einstein's theory to the evolution of the Universe, several theoreticians discover the possibility that the Universe is expanding or contracting. But Einstein dismisses this possibility because there was no evidence that the Universe is in motion. He believed the Universe is static, and proposes the existence of a hypothetical "repulsive force," called the cosmological constant that prevents galaxies from falling together.
1920 – 1930: Astronomer Edwin Hubble discovers Cepheid variable stars in several nebulae. These nebulae, he concluded, are galaxies far outside our Milky Way Galaxy, and that they were similar in size and structure to our Milky Way.
Astronomer Vesto Slipher makes measurements of the velocities of spiral nebulae, which shows they are all receding from Earth, but he does not realize they are remote galaxies.
In 1929, Hubble made another startling discovery: The more distant the galaxy from Earth, the faster it moves away. Hubble discovered a correlation between the distance of a galaxy and its recession velocity. This relationship is called the Hubble law and the relationship between the distance and velocity is known as the Hubble Constant. Both theories have helped astronomers better understand the evolution of the Universe. Astronomers need an accurate value for the Hubble Constant to estimate the size and age of the Universe.
1930 – 1950: Hubble's observations lead to the realization that, in a uniformly expanding universe, galaxies would have been closer together in the past. Early in the Universe, the density (and temperature) of matter would have been very high. This leads to a model for the evolution of the Universe, called the Big Bang theory. The theory says that the Universe began in an extremely hot and dense state and has been expanding and cooling ever since then. To test and constrain the Big Bang theory, astronomers work on making solid measurement of the expansion rate (needed to determine the size and age) and check this against an independent estimate based on the ages of the oldest stars in the Universe.
1950s: Before calculating an accurate value for the Hubble Constant, astronomers try to fine tune the cosmic distances. In 1952, Carnegie astronomer Walter Baade finds that the distance scale to galaxies is wrong because of an error in the luminosity scales of stars.
1960s: Astronomers detect the cosmic microwave radiation left over from the Big Bang, as predicted by theory.
Measurements of the density of light elements (such as hydrogen and helium) in the early universe also provide support of the Big Bang theory.
1970s: In the mid-1970s, Carnegie astronomer Allan Sandage discovers that some stars used by Edwin Hubble to estimate distances weren't as bright as once thought.
Though, distances to the nearest galaxies have been measured using Cepheids and other methods, unfortunately, astronomers cannot see Cepheids in distant galaxies. NASA begins construction on Hubble Space Telescope. One of the primary goals is to find Cepheids in more distant galaxies, opening the way to pin down an accurate value for the Hubble Constant.
1980s: Carnegie astronomer Wendy Freedman and Caltech astronomer Barry Madore conclude that dust in the spiral galaxies where Cepheids are located, significantly dims and reddens these stars, causing an error in the distance scale.
Astronomers refine 'secondary" methods for measuring the relative distances among galaxies. Among them are measuring the brightnesses and rotational velocities of entire galaxies and the measurement of another class of younger, more massive supernovae (exploding stars). Relative distances, however, do not alone provide a measure of the Hubble Constant. The situation is like the case of a road map with no scale printed on it. Two cities may be closer to each other than to a third city. Without a scale, no one will know the actual distances between those cities. Similarly, to measure the Hubble Constant, astronomers must know the actual distances to galaxies. Following the road map analogy, if the actual distance between two cities is known, then the actual distances among all other cities are established. Cepheids provide the absolute distance scale for celestial objects.
1990s:Using the Hubble Space Telescope, 14 internationally-based astronomers move toward pinning down the Hubble Constant. The astronomers' proposal, called the "Key Project on the Extragalactic Distance Scale," has three goals. The first is to measure Cepheid distances to about 20 galaxies and calibrate five secondary methods for measuring the relative distances to galaxies. The second is to measure Cepheid distances to galaxies in two of the nearest massive clusters of galaxies, Virgo and Fornax. The third is to check for errors in the Cepheid distance scale.
