How much do we know about the universe: The past 50 years have been a golden age for cosmology, the study of the origin and evolution of the universe. In the 1960s, a solid foundation was laid that would transform cosmology from a field dominated by speculation to a real science. The key figure in this transition was James Peebles, whose decisive discoveries put cosmology firmly on the scientific map.
A brief history of cosmology
It has only been in the last hundred years that we learned about the history of the universe, and even the fact that the universe has a history. Before this period, the universe was seen as immutable and eternal, until the 1920s astronomers discovered that all galaxies move away from each other and away from us. The universe is expanding, and we now know that today’s universe is different from yesterday and it will be different tomorrow.
What the astronomers saw in the sky had already been predicted by Albert Einstein’s 1916 theory of general relativity. This theory forms the basis for all large-scale calculations of the universe. When Einstein discovered that the universe is expanding, he added a constant to his equations, the cosmological constant. This constant counterbalances the effects of gravity and causes the universe to stand still.
More than 10 years later, when the expansion of the universe was observed, this constant was no longer needed. Einstein considered this matter the greatest blunder of his life. He had no idea at all that the cosmological constant would make a brilliant return to cosmology in the 1980s.
The first rays of light in the universe
The universe’s expansion means that it was once much denser and hotter. In the mid-20th century, the birth of the universe was called the big bang. No one knows what just happened at the very beginning. However, it is clear that the early universe was a dense, hot and opaque soup of particles. In this primordial soup of particles, photons, just bounced around.
It took nearly 400,000 years for expansion to cool this primordial soup to a few thousand degrees Celsius. The original particles were now able to fuse with each other, forming a transparent gas consisting mainly of hydrogen and helium atoms. Photons now began to move freely and light was able to travel through space. These first rays still fill the universe. However, the expansion of spacetime has stretched the rays of visible light to end up in the range of the invisible microwaves.
Cosmic microwave background radiation
That glow of the birth of the universe was first caught by chance in 1964 by two American radio astronomers. The 1978 Nobel Prize laureates Arno Penzias and Robert Wilson couldn’t get rid of a constant ‘noise’ that their antenna was picking up from all over space. So, they looked for an explanation in the work of other researchers who had made theoretical calculations about this ubiquitous cosmic microwave background radiation (CMBR).
After nearly 14 billion years, the temperature of the radiation had dropped to close to the absolute minimum – 273 degrees Celsius. A big breakthrough came when Peebles realized that the temperature of the radiation could provide information about how much matter had been created in the big bang. He understood that the release of this light played a decisive role in how matter clamped together to form galaxies and galaxy clusters.
New era in cosmology
The discovery of CMB ushered in a new era in cosmology. The CMBR has now become a gold mine containing the answers to almost everything cosmologists want to know. How old is the universe, what is its fate, and how much matter and energy exist in the universe?
Scholars can find traces of the very first moments of the universe’s existence in this cold afterglow. Tiny variations that propagated like sound waves through that early primordial soup. Without these minor variations, the cosmos would have cooled from a hot fireball to a cold and uniform emptiness.
We know that this has not happened, that space is full of galaxies, which often co-exist in clusters of galaxies. The background radiation is flat in the same way as the ocean’s surface is. Up close you can see the waves, ripples that reveal the variations in the early universe. Time has led the way in interpreting those fossil traces from the earliest eras of the universe. With astonishing accuracy, cosmologists were able to predict variations in the background radiation and show how they affect matter and energy in the universe.
The first major breakthrough came in April 1992, when researchers from the US COBE satellite project presented an image of the first rays of light in the universe. John Mather and George Smoot earned the 2006 Nobel Prize in Physics for this discovery. Other satellites, the American WMAP and the European Planck satellite, gradually refined this portrait of the young universe. Just as predicted, the otherwise uniform temperature of the background radiation varied by one hundred thousandth of a degree. With increasing accuracy, the theoretical calculations of the matter and energy contained in the universe were confirmed. And the vast majority of that, 95%, is invisible to us.
Dark matter and dark energy
Since the 1930s we have known that there is more than we can see. Measurements of the rotation speeds of galaxies indicate that they must be held together by the gravitational pull of invisible matter or else they would be torn apart. This dark matter was also thought to have played an important role in the formation of galaxies long before the primordial soup released its grip on the photons.
The composition of dark matter remains one of the greatest mysteries in cosmology. Scientists have long believed that the already known neutrinos could form this dark matter. However, the incredible numbers of low-mass neutrinos traversing space at a speed approaching that of light are far too fast to help put matter together. In 1982, Peebles proposed that heavy and slow particles of dark matter could get the job done instead of the neutrinos. Cosmologists are still looking for these unknown particles which avoid interacting with known matter and make up 26% of the universe.
The shape of spacetime
According to Einstein’s general theory of relativity, the geometry of the universe is linked to gravity. The more mass and energy the universe contains, the more it is curved. At a certain critical value of mass and energy, the universe will no longer be curved. This kind of universe, in which two parallel lines will never intersect, is usually called flat. Two other possibilities are a universe in which there is too little matter, which leads to an open universe in which parallel lines will eventually diverge. Or a closed universe with too much matter, in which parallel lines will eventually intersect.
Measurements of the CMBR and theoretical considerations gave a clear answer to this question: the universe is flat. The matter it contains only accounts for 31% of that critical value, of which 5% is ordinary matter and 26% is dark matter. Most, 69%, is missing. In 1984 James Peebles contributed to the revival of Einstein’s cosmological constant, which is the energy of empty space. This was called dark energy and it fills 69% of the universe. Together with the cold dark matter and the ordinary matter, that is enough to support the idea of a flat universe.
Is the universe expanding or shrinking?
Dark energy remained only a theory for 14 years, until the accelerating expansion of the universe was discovered in 1998. This discovery earned Saul Perlmutter, Brian Schmidt and Adam Riess a Nobel Prize in Physics in 2011. Anything other than matter must be responsible for the rapid expansion: an unknown dark energy must exert pressure. Suddenly a theoretical addition became a reality.
Both dark matter and dark energy are now among the greatest puzzles in cosmology. They only identify themselves through the impact they have on their environment – one pulls, the other pushes. For the rest, not much is known about it. What secrets are hidden in this dark side of the universe? Are there new laws of physics hidden behind the unknown? What else will we discover in our efforts to solve the riddles of space?
Most cosmologists now agree that the model of the big bang is a true story of the origin and development of the universe. Despite the fact that we only know 5% of its matter and energy. This tiny particle of matter eventually clumped together to make everything we see around us: stars, planets, trees, flowers and animals, as well as humans. Are we the only ones looking at the cosmos? Is there life elsewhere in space, on a planet orbiting another sun? No one knows. However, we now know that our sun isn’t the only one with planets. Most of the hundreds of billions of stars in the Milky Way should also have companion planets.
Astronomers are now aware of more than 4,000 exoplanets, and we have discovered strange new worlds that are nothing like our planetary system. The first exoplanet discovered with certainty was so strange that hardly anyone believed it to be true. The planet was too big to be so close to its host star.
Planet 51 Pegasi b
Michel Mayor and Didier Queloz announced their sensational discovery at a conference of astronomers in Florence, Italy, on October 6, 1995. It was the first exoplanet proven to orbit a star similar to our sun. Planet 51 Pegasi b, orbits rapidly around its star, 51 Pegasi, which is about 50 light years from Earth. The planet takes only a little over 4 days to orbit the star completely. This means that a year on the planet is four days long, and it is very close to the star – at a distance of only 8 million kilometers. The star heats the planet to more than 1,000 degrees Celsius. Things are much slower on Earth, with an orbit that takes a year at a distance of 150 million kilometers.
The newly discovered planet also turned out to be surprisingly large. A gaseous ball similar to the largest gas giant in our solar system, Jupiter. Jupiter’s volume is 1,300 times larger than Earth’s, and Jupiter weighs 300 times more. According to earlier ideas about planetary formation, planets the size of Jupiter should have formed far from the host star. Thus they would have to orbit the star a long time later. Jupiter has completed a full orbit around the sun for almost 12 years, so the short orbit of 51 Pegasi b came as a complete surprise. They had looked in the wrong places until then.
The discovery of additional exoplanets
Almost immediately after the announcement, two American astronomers, Paul Butler and Geoffrey Marcy, pointed their telescopes at the star 51 Pegasi. Soon they were able to confirm the revolutionary discovery of Mayor and Queloz. Just months later, they found two new exoplanets orbiting a sun-like star. The planets’ short orbital times came in handy for those astronomers who didn’t have to wait months or years to see an exoplanet make a full orbit around its star. They now had time to observe the planets as they completed one orbit after another.
How did those planets get so close to the star? That question challenged the existing theory of the origin of planets. Eventually, it led to new theories describing how gas giants were created at the edges of their solar systems and then spiraled toward the host star.
Methods that led to the discovery exoplanets
To find an exoplanet, advanced methods are needed: after all, planets do not emit light themselves, they only reflect the light of the star so faintly that their glow is overwhelmed by the bright light of their host star. The method used by research groups to find a planet is called Doppler spectroscopy. It measures the motion of the host star as it is affected by the gravity of its planet. As the planet moves around the star, the star also moves slightly – they both move around their common center of gravity. Viewed from the observation point on Earth, the star waddles forward and backward in line of sight.
The speed of that movement, the radial velocity, can be measured thanks to the familiar Doppler effect: rays of light from an object moving towards us are bluer, when the object moves away from us they are redder. We can hear the same effect when an ambulance’s siren goes higher when it approaches us, and lower when the ambulance has passed and is moving away from us.
The effect of the planet thus alternately changes the color of the light from the star towards blue or red, and it is these changes in the wavelength of the light that the astronomers receive with their instruments. The changes in color can be precisely determined by measuring the wavelengths of the star’s light, which gives a direct measure of its speed in the line of sight.
Shortcomings of Doppler spectroscopy
The biggest challenge is that the radial speeds are extremely low. For example, the gravitational force of Jupiter makes the sun move at a speed of about 12 meters per second around the center of gravity of our solar system. The Earth contributes only 0.09 m / s, which places extraordinary demands on the sensitivity of the instruments if one wants to discover an Earth-like planet. To increase accuracy, astronomers measure several thousand wavelengths simultaneously. The light is divided into the different wavelengths with the help of a spectrograph, which forms the heart of these measurements.
In the early 1990s, when Didier Queloz started his career as a researcher at the University of Geneva, Michel Mayor had spent many years studying the motions of stars, building his own measuring instruments with the help of colleagues. In 1997, Mayor was able to mount his first high resolution spectrograph on a telescope at the Haute-Provence Observatory. That allowed us to observe a lower limit of speeds of around 300 m / s, but not enough to see a planet pulling on its star.
Together with the research group, doctoral student Queloz was asked to develop new methods for even more accurate measurements. The researchers used new technologies that made it possible to quickly look at many stars and analyze the results on the spot. Fiber optic cables were able to direct the light from the stars to the spectrograph without distorting it, and better digital image sensors increased the machine’s light sensitivity. More powerful computers allowed the researchers to develop custom software for processing digital images and data.
In the spring of 1994, a new spectrograph called ELODIE was ready, causing the necessary speed to drop to 10 to 15 m / s. The first discovery of an exoplanet would not be long in coming. At the time, the search for exoplanets was not part of mainstream astronomy, but Mayor and Queloz had decided to announce their discovery. They spent several more months fine-tuning their results, and by October 1995, they were ready to present their very first planet to the world.
Thousands of new worlds brought to light
The discovery of an exoplanet orbiting a sun-like star revolutionized astronomy. Thousands of new worlds have since been discovered, and new planetary systems are now detected not only with Earth-based telescopes, but also with satellites. TESS, an American space telescope, is currently scanning more than 200,000 of the stars closest to us, looking for planets similar to Earth. Previously, the Kepler space telescope had already yielded significant loot with the discovery of more than 2,300 exoplanets.
Along with the variations in radial velocity, transition photometry is now also used to search for exoplanets. This method measures changes in the intensity of the star’s light as a planet passes in front of the star and obscures part of it, as it occurs in our line of sight. Transition photometry also allows astronomers to observe the exoplanet’s atmosphere as light from the star passes through it on its way to Earth. Sometimes the two methods can be used simultaneously. Transition photometry gives the size of the exoplanet, while its mass can be determined using the radial velocity method. With these two data it is possible to determine the density of the exoplanet and thus determine its structure.
The exoplanets discovered so far have amazed us at the mind-boggling variety of shapes, sizes and orbits. They have questioned our preconceived notions of planetary systems and forced researchers to rethink their theories of the physical processes responsible for the formation of planets. And with numerous projects underway to begin searching for exoplanets, we could eventually find an answer to the perennial question of whether there is other life somewhere in space.