How do we know that the universe is expanding and accelerating?
A short explanation
By Dirk Goës BSc(CompSc) UOW, MSc(Astronomy) Swinburne
About me: I have been a member of the Sydney City Skywatchers for two years. I am a member of the committee and am responsible for maintaining and improving the Skywatchers website. I recently graduated with a Master of Science (Astronomy) from Swinburne University of Technology.
This article is based upon the following lecture: ‘Why is the Universe accelerating? Nobel Laureate Brian Schmidt explains’ which is available at https://www.youtube.com/watch?v=55pcpTjd3BY
The universe is expanding and doing so at an accelerating rate. The scientific story of how this was determined took shape over many decades of observation and research by astronomers across the world, with each small discovery laying the path to the big breakthroughs.
The first big breakthrough came when Edwin Hubble measured the movement of multiple galaxies in relation to Earth out to approximately 1.3 billion years in the past. He found that the further away a galaxy is the faster it is moving away from Earth. This led to the conclusion that the universe is expanding.
The second big breakthrough came when Brian Schmidt, Adam Riess and Saul Perlmutter were able to measure the movement of galaxies out to approximately 4 – 5 billion years in the past. These more distant galaxies were also moving away faster than closer galaxies however at a slower speed than predicted by Hubble’s results. This led to the conclusion that the universe was expanding more slowly in the past and that therefore the expansion of the universe is accelerating.
This article gives a short explanation of how the expanding and accelerating universe was discovered and the related and unsolved mysteries of dark energy and dark matter.
The expanding universe
When an ambulance is travelling towards you at high speed the sound waves from the siren get compressed and therefore the pitch of the siren is higher. However, when the ambulance is travelling away from you, the sound waves get stretched and the pitch of the siren is lower. In the study of sound waves this effect is called the doppler shift.
Light also behaves as a wave and the same effect applies. When an object such as a star or galaxy is moving towards Earth the light waves get compressed and become bluer. When a star or galaxy is moving away from Earth the light waves get stretched and become redder. In the study of light waves this effect is called redshift.
Astronomers can measure the speed and direction of an object by comparing the redshift of the observed light to the light as it was originally emitted. It is possible to do this because when the light is broken up into a spectrum it contains a fingerprint that allows this comparison to be made.
This fingerprint consists of dark lines in the spectrum called absorption lines. The absorption lines are caused by the light interacting with different elements such as hydrogen and helium that makeup the gas that the light travels through to reach Earth. This could be the gas in the atmosphere of a star or the gas in and around a galaxy. The absorption lines for specific elements always appear at the same wavelengths when measured in a laboratory on Earth. In the spectrum of a star or galaxy these absorption lines will be either shifted to the blue end of the spectrum or shifted to the red end of the spectrum. By comparing the position of these absorption lines to the laboratory lines, the speed and direction of the object can be calculated.
In 1916 Vesto M. Slipher was the first astronomer to discover that the light of most galaxies he observed was shifted towards the red end of the spectrum. In 1929 Edwin Hubble was the first astronomer to be able to measure the distance to the galaxies that Slipher had observed. He was able to do this by observing a special class of star called a Cepheid variable. A Cepheid variable is a type of Red Giant star that pulsates. The pulsations cause the star to brighten and dim repeatably over a predictable number of days. In 1912 Henrietta Swan Leavitt discovered the relationship between the brightness of a Cepheid variable and the number of days it takes for it to brighten and dim (one pulsation). This relationship allows the distance to the star to be calculated using the inverse square law of light. The inverse square law states an object becomes fainter by the square of its distance. Or that if a light source is twice as far away it will be four times fainter. This allows the distance to be calculated by comparing the observed brightness to the known brightness.
Hubble was able to make use of this calculation by observing Cepheid variables in each galaxy he was interested in measuring the distance to. By correlating the speed and direction of each galaxy (calculated from redshift) with the distance, Hubble found that not only were most galaxies moving away from Earth but that the further away a galaxy was, the faster it was moving away. This led Hubble to conclude that the universe is expanding.
It is important to note that this finding does not imply that the Earth is at the centre of the universe and all galaxies are moving away from only us. The same finding applies to any point in the universe from which similar observations are made.
The accelerating universe
Subsequent observations by other astronomers confirmed Hubble’s results. However, observations that rely on Cepheid variables could only measure the distance to galaxies which are relatively close to Earth on astronomical scales. Beyond that Cepheid variables become too faint to observe.
When astronomers observe distant galaxies, they are looking back in time as the further away a galaxy is, the longer the light has taken to reach Earth. The observations made by Hubble and others using Cepheid variables allowed astronomers to look back about 1.3 billion years into the past. However, this is only a fraction of the estimated 13.8-billion-year age of the universe. Another technique to measure even more distant galaxies was required to determine whether Hubble’s results hold true in the distant past.
From the mid-1930s onwards astronomer Fritz Zwicky started to observe a class of objects known as Type Ia Supernovae which were much brighter than Cepheid variables but also appeared to be consistent in their brightness no matter where or how far away they occurred.
A Type Ia Supernova is a type of exploding star that results from the interaction of two stars that are closely orbiting each other (a binary system). The older of the two stars which has evolved into an object called a White Dwarf sucks material off its younger companion which has expanded into a Red Giant star. Once the White Dwarf has collected a critical mass of material from its companion a thermonuclear explosion occurs completely destroying the White Dwarf. This explosion is several billion times brighter than our Sun.
Over several decades of observing, Fritz Zwicky and his colleagues were able to use Type Ia Supernovae to match Hubble’s results of an expanding universe even so the errors and uncertainties in the results were quite large.
Subsequently in the early 1990’s a research group known as the Calan-Tololo Supernova Search were able to dramatically improve the accuracy of Type Ia Supernovae observations. They did this by discovering that not all Type Ia Supernova are exactly the same but rather that there is a relationship between the brightness of the explosion and the amount of time it takes for the explosion to brighten and subsequently fade away. This led to a much more accurate confirmation of Hubble’s results.
With Type Ia Supernova established as a very accurate way of measuring distance together with their incredible brightness, the stage was now set for measuring the recessional speed of much more distant galaxies beyond 1.3 billion years ago. The final ingredient that was needed was improved observing technology which came along with the construction of the 10-metre Keck telescopes in Hawaii, and the development of very sensitive and high-resolution CCD (Charged Couple Device) cameras.
During the 1990s two teams took advantage of these developments and started to search for very distant Type Ia Supernova. They were the High-Z Team led by Brian Schmidt & Adam Riess and the Supernova Cosmology Project led by Saul Perlmutter. Both teams were able to identify Type Ia Supernova beyond 1.3 billion years in the past out to between 4-5 billion years in the past.
To their amazement both teams found that the distant supernova had smaller redshifts than expected. This meant that they were moving away from Earth at a slower than expected speed compared to that predicted by the closer supernova and the galaxies in Hubble’s results. This indicated that the universe was expanding more slowly in the past. This finding was so unexpected that a lot of follow up work was done to eliminate any mistakes or alternative explanations, but in the end the results stood up.
To understand the implications of this finding, the teams had to compare their results with models of the universe built by cosmologists. Cosmologists are astronomers who specialise in understanding the origin and future of the universe. Importantly cosmologists had already measured the oldest light in the universe, the Cosmic Microwave Background (CMB). The CMB has revealed that we live in a universe that is geometrically flat. That is, two beams of light travelling in parallel across billions of light years will never converge or diverge assuming that nothing gets in their way.
Based on the CMB, Einstein’s theory of general relativity as well as other types of observations, cosmologists have built a model of the flat universe that shows under what conditions it would be contracting or expanding at either a steady, accelerating, or decelerating rate. The supernova observations made by the High-Z Team and the Supernova Cosmology Project fit into the model where the universe is expanding at an accelerating rate. This phenomenal and unexpected result led to both teams to being awarded the Nobel prize for physics in 2011.
The makeup of the universe
The obvious question then is what is causing this accelerated expansion? The universe started with the Big Bang, and it is reasonable to imagine that the ongoing effect of this ultra powerful event is continued expansion, but what would be causing it to accelerate?
The answer is we do not know and therefore cosmologists have labelled this accelerating force as Dark Energy. Dark because we cannot see or detect it. Cosmologists hypothesise that dark energy is the energy of space itself and that this energy is causing space itself to expand. Therefore, the distant galaxies that are all moving away from Earth are not moving away through space, but rather expanding space is pushing the galaxies further apart.
Conversely, we do not see this accelerated expansion happening in our corner of the universe. For example, the Sun and Moon are not accelerating away from Earth. Our home galaxy of the Milky Way also shows no signs of being stretched apart. This is because where there are many massive objects close together the attractive force of gravity is more powerful than the expanding force of dark energy. In fact, this extends out to other galaxies which are relatively close to us on astronomical scales. For example, the Andromeda galaxy, the closest large neighbour to the Milky Way, is moving towards us.
It appears then that gravity holds clusters of galaxies together while dark energy increases the amount of space between galaxy clusters. This leads to a further mystery because when astronomers measure the amount of luminous matter (i.e. the stuff we can see) in a galaxy cluster, it turns out there is not enough mass to create the gravitational force to hold it together. This means there is more matter present than we can detect and cosmologists have labelled this material as Dark Matter.
Matters get stranger still because when cosmologists use the geometrically flat universe to calculate the average mass density of the universe, they find that most of the universe consists of dark energy (~68%) with a smaller portion consisting of dark matter (~27%) and normal matter (~5%). Normal matter is made from atoms and consists of everything we can see and measure including galaxies, stars, planets, the Earth, trees, animals and humans.
Unravelling the mysterious universe
We live in a mysterious universe and despite the incredible scientific discoveries we have made we have yet to understand the nature of most of the material and energy that makes it up. However, the hunt for dark energy and dark matter is on with many facilities and institutions around the world collecting data and conducting intensive research.
For example, the Dark Energy Spectroscopic Instrument (DESI) installed on the Kitt Peak National Observatory in the United States is collecting observations to build a 3D map of the universe out to 11 billion years in the past. This map will be used to study how structures in the early universe are reflected in universe at different time periods up to the present. The evolution of these structures can help determine the behaviour and nature of dark energy.
In a similar fashion, the Square Kilometre Array radio telescopes being built in South Africa and Australia will be able to build a map of hydrogen distribution from the early universe up to today to study the influence of dark energy on this distribution.
Dark matter is being searched for in underground laboratories around the world. Underground labs shield the detection instruments from unwanted interference. For example, a new facility is currently under construction one kilometre underground in a gold mine near the town of Stawell in Victoria, Australia. This facility will co-ordinate its experiments with a similar detector installed in the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, to attempt to confirm possible dark matter detections made in the past at the Italian facility.
Conclusion
This article has discussed how a series of important discoveries led Edwin Hubble to observe and conclude that the universe is expanding and how this discovery combined with new techniques and technology led Brian Schmidt, Adam Riess and Saul Perlmutter and their teams to conclude that not only is the universe expanding but that expansion is also accelerating. It has also discussed that we do not yet understand the nature of the force that is causing the accelerating expansion dubbed Dark Energy. Nor do we yet understand the nature of a large proportion of the matter (dubbed Dark Matter) that holds galaxies and galaxy clusters together. Finally, the article described some examples of current experiments being conducted to help solve these mysteries.
References
Hubble’s original paper presenting the evidence for an expanding universe:
Hubble, Edwin., 'A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae', Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, March 1929 pp. 168-173 https://ui.adsabs.harvard.edu/abs/1929PNAS...15..168H/abstract
The original papers from the High-Z Team and the Supernova Cosmology Project presenting the evidence for an accelerating universe:
Riess, Adam G. et al., 'Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant', The Astronomical Journal, Volume 116, Issue 3, September 1998, pp. 1009-1038 https://ui.adsabs.harvard.edu/abs/1998AJ....116.1009R/abstract
Perlmutter, Saul. et al., 'Measurements of Ω and Λ from 42 High-Redshift Supernovae', The Astrophysical Journal, Volume 517, Issue 2, June 1999, pp. 565-586 https://ui.adsabs.harvard.edu/abs/1999ApJ...517..565P/abstract
Links to the observing facilities mentioned in article:
W. M. KECK OBSERVATORY https://www.keckobservatory.org/
Dark Energy Spectroscopic Instrument (DESI) https://www.desi.lbl.gov/the-desi-science-mission/
Stawell Underground Physics Lab https://www.supl.org.au/
댓글