The Expansion of the Universe
In recent years, astronomers have been more accurate when it comes to calculating how fast the universe is expanding over the past 14 years. However, the more accurate the numbers become, the more complex the idea becomes.
There are two main methods of calculating the speed of the expansion of the universe. The first is observing astrophysical objects such as stars and supernovas. The other is using the laws of physics to extrapolate information about the age of the universe. The values that we get from both of them should be the same, right? Well, that is not the case. To understand how this can possibly happen, we need to redefine how the universe is expanding. The current definition is the void between the galaxies and other large objects. Nonetheless, the void may not be expanding at equal rates; in other words, the universe is not expanding everywhere.
One of the methods of measuring this expansion requires calculating the distances to stars called Cepheid Variables. A Cepheid is a star whose brightness changes over very regular periods of time. The length of that period is directly related to how bright the star is. Thus, as long as scientists can measure how fast these objects change, they can figure out how bright they are up-close. Then, they can compare that number to how bright the star looks from Earth to determine the distance. From all of this, it is possible to figure out how fast the universe is expanding. There are a few other ways of doing this but the Cepheid variable was the most suitable since the researchers were able to use the Hubble Space Telescope to look at 70 Cepheids in a nearby dwarf galaxy called the Large Magellanic Cloud. This is only around 162,000 light years away which is pretty close compared to the whole universe.
The data from the Cepheids was then combined with another set of data obtained by the Araucaria Project. The aim of this project was to watch the light of binary star systems change as the stars moved around one another. That movement allowed them to figure out the stars’ masses and how big they are. Then, by combining that with the data about how fast those changes happened and what kind of light stars emitted, the scientists could ultimately work out how far away they are.
From all of the various calculations done from the observable universe, the researchers came to the conclusion that the universe is expanding at 74.03 kilometers per second per megaparsec. This means that an object one million parsecs away or roughly 3.3 million light years is moving away from us at about 74 kilometers per second. An object two million parsecs away is moving away at about 148 kilometers per second. Nevertheless, the estimate conflicts with other confident measurements about the universe’s expansion.
The Large Magellanic Cloud
Another method of finding out the rate at which the universe is expanding is by studying the Cosmic Microwave Background. This is the oldest light in the universe that humanity will ever see and dates back to when the cosmos was only about 380,000 years old. Studying it is the main objective of the European Space Agency’s Planck Telescope. By studying temperature fluctuations in this light, scientists have been able to determine how fast the universe was expanding 13 billion years ago. With that information, it is possible to extrapolate and figure out what the expansion rate should be today. The extrapolations are all based on well-tested laws of physics and in theory, the result here should match up with the data from before by Hubble; however, as said before, it does not.
The Planck expansion rate is noticeably lower than what we have gotten using sources like Cepheids, only around 67.4 kilometers per second per megaparsec. What is more surprising is that the chance of either number being incorrect is 1 in 100,000. Hence, it is unlikely that the numbers are wrong.
Why is the observed expansion rate 10 percent faster than what physics predicts it should be? There are several theories to this. The first hypothesis is where dark energy caused an increase in the universe’s expansion rate. The scientists do not really know what dark energy is but they believe something like this has already happened twice, once for a brief moment after the big bang and again starting a few billion years ago. It is possible that there was another incident like this between the two points.
Another hypothesis is that dark matter interacts differently with regular matter and light than we think. Dark matter is stuff that doesn’t interact with light or charged particles and is therefore invisible to us. We only know that it exists because of the gravitational effect it has on mater and light. The theory suggests that we could be wrong about how strong its influence is on matter. If the influence is indeed stronger, it could have countered the universe’s expansion early-on.
Both hypotheses could be wrong and maybe there is some exotic particle that we have not yet discovered that is responsible for all of this. The scientists could also explore this further by using gravitational waves produced in a black hole and neutron star-merge. Since they do not rely on light, measuring those waves would give us a totally new set of data to study the expansion rate. Nevertheless, this field is still young and not much is known about this.
Understanding the rate of expansion of the universe is all about discovering and understanding the fundamental rules for how everything works. With a better understanding of the universe, we will eventually be able to explain Cepheids that are way out in space to gravity that keeps us on Earth.
 |  |  |
|---|