Science Background
Closing in on Understanding the Expanding Universe

What'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.