Cosmological expansion is a recent breakthrough in our understanding of the universe. It began with the observation that distant galaxies (each containing billions to trillions of stars) found in every direction from our own Milky Way are accelerating away from one another. The light from them was redshifted indicating that they are traveling away from us.
The universal feature of cosmological expansion only became discernible once we began to look at the universe from a greater perspective, attempting to analyze it as a whole.
It was found that the apparent velocity of an object increased as it moved away from us, at a rate linearly proportional to its distance from us. As galaxies receded further into the universe, they accelerated at a constant rate resulting in linear increase in velocity with distance.
A Big Bang & The Expanding Universe
This rate of recession has been understood as the rate of the Expansion of the Universe. It is now known as the Hubble Law, named after Edwin Powell Hubble, the famous American Astronomer who discovered it. (The notable Hubble Space Telescope also bears his name.)
One of the first conclusions we can draw from the Hubble Law is the age of the universe. If a galaxy is travelling at a certain velocity away from us, and if its velocity increases at a constant rate with distance, then we can logically conclude that at an earlier point in time that distant galaxy would have been closer to our galaxy. At which point in time it would also have been travelling at slower speeds.
The further we go back in time the closer all galaxies come to one another. We can ultimately proceed in this way to what we might call the beginning of time, an initial state when all matter begins to collapse together into a single point. Since rate of cosmological expansion appears to have been uniform we can calculate the amount of time that has passed since this coalescence using the known rate of universal acceleration. In this way we have arrived at a date of 13.7 billion years since all matter was once together as a singular unified whole, the Age of the Universe.
Confirmation For Cosmological Expansion
The Big Bang Theory is the embodiment of the understanding that the origin of all matter can be traced backwards in time towards a singular point of origin. This idea was a logical conclusion derived from the discovery that all galaxies are accelerating away from us now.
We traced this constant rate of acceleration to a primordial event in which spacetime literally exploded in a flash of light. This event created the lightest of particles (hydrogen and helium at a ratio of roughly 3:1 or 75%-to-25% if you prefer) which is how matter was initially created.
Over the billions of years since matter then collapsed into interesting states dictated by mass and fundamental laws of the universe. Matter coalesced gravitationally and at a certain mass, pressure, temperature, and density caused matter to ignite as stars under the pressure of gravity.
Stars grouped in galaxies organized by the formation of a black hole at the center, or else the aggregation of stars forms a black hole at their geometric center as they drifted towards one another binding into a mutual orbit. Throughout this time the entire universe was constantly undergoing a cosmological expansion, which we will more technically define as cosmic inflation later on.
This has been the most dominant cosmological theory so far, since the discovery of Cosmic Microwave Background Radiation which has generally been confirmatory. In 1964 Arno Penzias and Robert Woodrow were constructing a Dicke radiometer, a device used to measure electromagnetic radiation in the form of microwaves which exist at the cm to mm wavelength range. Such devices are predominantly used in radio astronomy.
In their very first measurement with their new radiometer Penzias and Woodrow detected a 4.2 K excess of antenna temperature. They could not explain this shift of 4 degrees. Little did they know at the time, but they were discovering a feature of the universe for which they would eventually be awarded the 1978 Nobel Prize in Physics.
Though CMBR had already been predicted as early as 1948, with initial estimates for the temperature of this radiation being around 5 K (very accurate) the radiometer of Penzias and Woodrow was the first physical confirmation. Radio telescopes literally listen to electromagnetic signals from space, as cosmic object emit all types of electromagnetic radiation. Scientists have since detected a microwave radiation coming uniformly coming from every direction of space, almost as a background noise.
This uniform Cosmic Microwave Background Radiation is believed to be the archaic remnant radiation of the original flash of light that radiated across the virgin universe with its emergence some 13.7 billion years ago. It is alternatively known as relic radiation, a relic from the brilliant universal explosion that began spacetime, sending matter hurtling from a single point across the expanding cosmos.
Cosmological Expansion & Cosmic Inflation
The cosmological expansion has now been interpreted as the literal expansion of spacetime. The literal space of the universe is inflating in size and geometry. Galaxies are not flying through space but rather are entrenched in position as a gravitational warp in spacetime while the space between these two points (such as galaxies) is moving away from one another at a constantly increasing velocity.
This is what we call cosmic inflation, a theory proposed in 1979 explaining why galaxies are redshifted and potentially clarifying elements of our understanding of the nature of the universe.
Though in our search for the truth we must remember that our observation of distant galaxies provides a picture of them as they were perhaps billions of years ago. Astronomy is a discipline that intrinsically looks back in time. All star light may take anywhere from 4 years to 100,000 years to reach us in our galaxy alone, depending on distance. The light from distant galaxies may have taken billions of years to reach us, which means that the position of the distant galaxy (and its apparent velocity and acceleration) is not anywhere near what it actually is at the moment. For all we know it could have been eaten by a giant monster a billion years ago. This is the Consequence of the Travel Time of Light.
We also must keep in mind Albert Einstein’s theory of General Relativity. This tells us, among other things, that the flow of time itself is altered by extreme mass. Mass is a warp in the fabric of spacetime, which causes time dilation. In practice this tells us that when the objects in the universe were closer together the universe would be more dense which means that time would flow slightly differently at this phase of the universe than it does now. In cosmology we must deal with these ridiculous magnitudes of mass, time, and distance very carefully.
Further Reading
- Popular Articles
- Cosmology | Frank H. Shu | Professor of Physics, University of California, San Diego | Encycopedia Britannica | Accessed 11 Sept 2020
- Expansion of the Universe | Wikipedia | Accessed 11 Sept 2020
- Technical Articles