A MEETING WITH THE UNIVERSE

Chapter 4-9

The Edge of the Universe

There is no place in the universe that is truly empty, but the space be tween clusters of galaxies comes close. These regions contain less than one atom in every 10 cubic meters (350 cubic feet), or only three atoms in a space about the size of a small room. Although galaxies continually supply new material to intergalactic space, the continuous expansion of the universe makes the net effect nil. Intergalactic space is very empty, and it is getting emptier as the universe expands.

The more we learn about it, the more complicated the expansion of the universe seems to be. In the region near our galaxy, the expansion seems less rapid than for the universe as a whole. In fact, it appears that the combined gravitational pull of a very large cluster of galaxies in the constellation Virgo is actually retarding the local rate of expansion to half the rate for the universe as a whole. We're finding evidence of how gravity attracts even over distances of hundreds of millions of light years. Although there must be many very distant galaxies and quasars that we are not yet able to detect, astronomers have observed radiation from an even more remote source, literally at the edge of the observable universe.

As we look far out in space, we are looking back in time, since light waves take time to cross space from their source to the observer. Hence, we view distant regions of the universe as they were long ago. According to the Big Bang theory, the universe originated in a great explosion and has been expanding ever since. At a very early epoch, before galaxies and stars formed, the universe was filled with hot glowing gas, and it was opaque. At some point during the first million or so years after the Big Bang occurred, the expanding and cooling gas became transparent. Hence, we can see out into space and back in time only until we come to the distant region that we observe as it was in the era when the universe cleared. Beyond that point, space is opaque so light waves cannot reach us. We see the glow from the hot gas that cooled to about 10,000° C and then cleared, but we cannot see further. This glow was emitted as ultraviolet light, but has been shifted to longer wavelengths by the expansion of the universe, so that we observe it today as a diffuse back ground of microwave (short-wave length radio) and infrared radiation. This microwave background is thus the radiation that comes to us from the limits of the observable universe.

We can never see beyond it to more distant regions or earlier times. Due to the redshift, the background radiation resembles the emission from a dense gas at only 2.7° C above absolute zero. You can think of the source of the microwave background as a distant, spherical wall that surrounds us and delimits the observable universe. If an observer is moving with respect to the wall, then the spectrum of the radiation coming from the region of the wall that he is approaching will be shifted toward shorter wavelengths, while radiation from the opposite direction will be shifted toward longer wavelengths. Thus, the existence of the microwave background allows us to determine whether the solar system is moving with respect to a basic frame of reference in the universe.

According to the results from recent measurements with telescopes on high-altitude aircraft, balloons, and sounding rockets, we appear to be moving at about 300 kilometers per second (190 miles per second) toward the constellation Leo. When we launch infrared and microwave telescopes on orbiting satellites, it will be possible to make more sensitive measurements and to search for structure in the spatial distribution of the microwave background that may reveal fundamental aspects of the nature of the universe.

diagram illustrating the difference in time of astral events and our perception

Out in space and back in time.
Light waves from distant space show how objects there looked when the waves left. Light from the nearest star outside the solar system takes over three years to reach the Earth, while photographs and X-ray images of very distant quasars show them as they were billions of years ago. Expansion of the universe shifts light waves to lower frequencies, but they carry information to us on the early history of the cosmos.