And, the galaxies weren’t standing still. The first act of this revelation came when, a century ago, American astronomer Edwin Hubble discovered myriad galaxies outside of our home galaxy, the Milky Way. Furthermore, we wouldn’t have any idea about its age either – or in fact, if the universe was eternal. Without any observational evidence for space expanding, contracting, or standing still, we wouldn’t have a clue as to whether the universe was coming or going. Science history will record that the search for the expansion rate of the universe was the great Holy Grail of 20th-century cosmology. Credit: NASA’s Goddard Space Flight Center Three Decades of Space Telescope Observations Converge on a Precise Value for the Hubble Constant More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. This idea stems from the observation that all galaxies seems to be receding from each other at an accelerating pace, implying that some invisible extra energy is at work.A representation of the evolution of the universe over 13.77 billion years. Even less is known about it than dark matter. Unlike stars and galaxies, dark matter does not emit any light or electromagnetic radiation of any kind, so that we can detect it only through its gravitational effects.Īn even more mysterious form of energy called “dark energy” accounts for about 70% of the mass-energy content of the universe. A very large fraction of the universe, in fact 26%, is made of an unknown type of matter called " dark matter". Astronomical and physical calculations suggest that the visible universe is only a tiny amount (4%) of what the universe is actually made of. Heavier atoms such as carbon, oxygen and iron, have since been continuously produced in the hearts of stars and catapulted throughout the universe in spectacular stellar explosions called supernovae.īut stars and galaxies do not tell the whole story. Present observations suggest that the first stars formed from clouds of gas around 150–200 million years after the Big Bang. These were mainly helium and hydrogen, which are still by far the most abundant elements in the universe. It took 380,000 years for electrons to be trapped in orbits around nuclei, forming the first atoms. As the universe continued to expand and cool, things began to happen more slowly. Within minutes, these protons and neutrons combined into nuclei. A few millionths of a second later, quarks aggregated to produce protons and neutrons. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. In the first moments after the Big Bang, the universe was extremely hot and dense. Though the Big Bang theory cannot describe what the conditions were at the very beginning of the universe, it can help physicists describe the earliest moments after the start of the expansion. Physicists had assumed that matter in the universe would slow its rate of expansion gravity would eventually cause the universe to fall back on its centre. This earned them the Nobel prize in physics in 2011. In 1998 two teams of astronomers working independently at Berkeley, California observed that supernovae – exploding stars – were moving away from Earth at an accelerating rate. Subsequent calculations have dated this Big Bang to approximately 13.7 billion years ago. Lemaître proposed that the universe expanded explosively from an extremely dense and hot state, and continues to expand today. Hubble’s discovery was the first observational support for Georges Lemaître’s Big Bang theory of the universe, proposed in 1927. If galaxies are moving away from us, reasoned Hubble, then at some time in the past, they must have been clustered close together. Hubble’s observation implied that distant galaxies were moving away from us, as the furthest galaxies had the fastest apparent velocities. Redshift occurs when a light source moves away from its observer: the light's apparent wavelength is stretched via the Doppler effect towards the red part of the spectrum. In 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts.
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