Expansion of the Universe
The expansion of the universe is the phenomenon by which space itself expands, gradually pushing galaxies farther apart from one another. This is not a movement of galaxies through space, but an expansion of space itself— like points drawn on a balloon that move away from one another as the balloon is inflated, with none of them at the center.
Hubble’s Discovery
In 1929, Edwin Hubble published a fundamental observation: all galaxies are moving away from us, and their recession velocity is proportional to their distance. This is Hubble’s law. This discovery, confirmed by redshift measurements, implies that the universe was smaller in the past—and that it must have from a very dense and very hot initial state. It constitutes the main observational evidence for the Big Bang model.
Accelerated expansion
In 1998, two independent teams studying distant supernovae discovered that the expansion does not slow down under theeffect of gravity—it was accelerating. This discovery, which was awarded the Nobel Prize in Physics in 2011, led to the introduction of the concept ofdark energy to explain this mysterious acceleration. Today, the universe is expanding faster and faster, irreversibly pushing distant galaxies beyond our observable horizon.
Importance in Astronomy
The expansion of the universe lies at the heart of all modern cosmology. It determines the relationship between the distance and redshift of galaxies, allows us to measure the age of the universe, and shapes its future fate. The precise measurement of the expansion rate— the Hubble constant—is one of the major challenges in contemporary astronomy, particularly due to the discrepancy between different measurement methods.
Concrete example
The galaxy NGC 7619, located about 200 million light-years away, ismoving away from us at about 3,800 kilometers per second due to the expansion of the universe. Even more distant galaxies are moving away even faster— some, beyond our cosmic horizon, are moving away from us at speeds greater than the speed of light, not becausethey are moving through space, but because space itself is expanding.
Frequently Asked Questions
If the universe is expanding, will galaxies eventually disappear?
Yes, over enormous time scales. Very distant galaxies are already moving away from us at speeds greater than the speed of light. In billions of years, due to this acceleration, more and more galaxies will cross our cosmic horizon andfrom our observable universe forever.
Does the expansion of the universe also apply to local objects?
No. On a local scale—planetary systems, stars, individual galaxies—gravity dominates and keeps objects bound together. Cosmic expansion applies only on large scales, where gravity no longer binds structures together. The Milky Way and Andromeda, for example, are moving closer together despite the general expansion.
Does the expansion of the universe create matter?
No, according to the standard model. Space is expanding, but the amount of matter remains constant. The density of matter therefore decreases over time. In steady-state theories (now abandoned), the continuous creation of matter was postulated to maintain a constant density despite the expansion.