How do we know what we know?

Seven is a good number – mysterious, magical and all that. Usually, it means something positive. Thus it seems appropriate for seventh to be the final instalment in this article series on the Universe.

In the last part, I wrote about how the Universe will end. Another way of wrapping up the series is by telling you how we know what we know now, what we still need to find out and how we are going to do that. So sit back and (hopefully) enjoy reading a little bit about the history of astrophysical discoveries.

It should be no surprise that people observed the skies for as long as they were self-aware. There is something deeply mysterious and breathtaking about the night sky, filled with tiny dots of light. People also wondered what it is that they see – spirits of dead warriors, heavenly fireflies or legendary tales personified in the skies above. These stories gave rise to the names of many stars and their groupings – the constellations, many of which have survived the centuries with few changes, if any.

 Records of astronomical information date as far back as writing in general. The first (quasi)scientific study of stars was carried out in ancient Greece, where a philosopher and astronomer Hipparchus catalogued a huge number of stars he could see in the night sky, defined the constellations and invented the magnitude system. The magnitudes, although a rather anachronistic system of measurement, are used in modern astronomy as well. Hipparchus defined them by what he could see – the few brightest stars in the night sky he gave a magnitude of zero; the stars that looked slightly fainter he gave a magnitude of one; those that looked as much fainter than magnitude one as magnitude one was fainter than the brightest ones became magnitude two; and so on, up to six, which is the faintest magnitude humans can see with naked eyes. This resulted in a logarithmic scale, because human eyes act as logarithmic receptors of light. Currently, a difference of one magnitude between two stars means that the one with a lower value has luminosity twice that of the other’s.

Not much progress has been made in astrophysics over the course of the next millennium. Mostl likely everyone had more important things to do, such as invading Troy, building Rome, conquering half of Europe, descending into anarchy, migrating from the steppes, reconquering Jerusalem for Christendom and so on. The geocentric world view was prevalent at the time, with the Sun, Moon, planets and stars rotating around us, fixed in the immeasurable void. Then around came Nicholas Copernicus and said “Who do you guys think you are?”, but in a more polite way, which is currently known as the Copernican, or the Cosmological, principle. It states that on the largest scales, the Universe is homogeneous (identical everywhere) and isotropic (identical in every direction). Of course, it was first used to claim that it is not the Sun that rotates around the Earth, but vice versa. The authorities did not like such thoughts and their proponents – Copernicus, Galileo Galilei and various others, were silenced. Not for long.

At around the same time, various new ideas were popping up in the European world. Some of them were brand new; some were recycled from the Muslim world or Antiquity. Newton developed his theory of motion and gravity. Rene Descartes, indirectly aided by many predecessors, formulated the basics of the scientific method. Telescopes were invented and used to study motions of the heavens. Science started marching forward.

Yet even though the heliocentric view was soon accepted and became mainstream, astronomy again lapsed into a secondary role among sciences. The reasons for this were twofold: first of all, it was difficult to make any significant observations with the telescopes of the time; more detailed scientific investigations of stellar objects could be made only when spectroscopy was invented. Secondly, astronomy is a very fundamental branch of science; its discoveries bring no direct profit and even indirect profits are few and far between; combined with usually large monetary investments required, this makes astronomical (or astrophysical) research an economic nightmare. It is only natural that little money was spent on such projects while the national treasuries were strained by colonial and military ventures of the 17th-19th centuries.

In the late 19th century, a renaissance of astrophysics started and continues up to the present day. It is difficult to pinpoint one event that started everything – perhaps there is none. However at that time scientific investigations, especially in fundamental sciences, experienced a huge surge in interest and funding. Naturally, many groundbreaking discoveries were made, changing our view of the Universe time and again.

Although no scientific discovery is the work of one person alone, some happen to go down to history and get a lot of credit. Two such people involved with our understanding of the cosmos were Albert Einstein and Edwin Hubble. The first of the two is most famous for his statement that matter and energy are equivalent, although his two other contributions to science are, arguably, more important: the photoelectric effect, for which he received a Nobel Prize; and the general theory of relativity, which, among other things, described the general shape of the space of the Universe. Hubble, on the other hand, was an observer. He confirmed the earlier reports that the strange spiral nebulae are moving away from us, proved that those nebulae are actually other galaxies, and formulated an eponymous law stating that the further an object is, the faster it recedes from us. The precise relation between speed and distance, the Hubble constant, has been subject to much debate and it was only in the last decade that a value of ~72 km/s/Mpc has been accepted by the majority of the scientific community.

The two “ends of the rope”, observational results and theoretical model, were put together by one Georges-Henri Lemaitre. He was a Catholic priest and an astrophysicist – a rather strange combination if he were alive today, isn’t it? – and pioneered the use of Einstein’s general relativity in cosmology. He derived the theoretical basis of Hubble’s law two years before Hubble provided observational evidence for such expansion. He also created the Big Bang theory, by proposing that if the Universe is expanding, it must have began as a tiny dot (Lemaitre called it “the primeval atom”).

For a few decades, the Big Bang theory was rivalled by the Steady State Universe theory, which said that the Universe has always been approximately the same and the apparent expansion is counteracted by spontaneous generation of hydrogen atoms in the vacuum. Theoretically, such an explanation could have been possible, but later observations showed this not to be the case. For one, with better detection tools, it was noticed that spontaneous generation of hydrogen does not happen, at least not on the rate necessary to counteract the reduction in average density due to expansion. In addition, the cosmic microwave background radiation was detected.

Theoretical work on the Big Bang theory showed that if the Universe was denser than some particular value, matter and radiation would be coupled. As the Universe expanded, radiation would decouple from matter and faint echoes of that coupled stage should be detectable even today. Proof that this was the case came completely accidentally in 1964, when two radio astronomers, Arno Penzias and Robert Wilson, discovered a faint microwave glow everywhere in the sky while preparing for some radio galaxy observations. Once they checked that the glow is real rather than just a fluke in their telescope, they soon learned of the recent predictions of such a radiation. The Big Bang model was vindicated.

Fast forward a decade, to the mid 1970s. While the Big Bang was the accepted theory of how the Universe began and evolved, it was far from perfect. Two glaring problems were the flatness problem and the horizon problem. The first one has to do with the so-called “flatness” of space. Our Universe, as far as we know, is perfectly flat. However, there seems to be nothing precluding it from forming as nonflat, and any curvature actually increases as space expands (I won’t go into mathematical details). Therefore in order for the Universe to be flat, it had to form perfectly flat, which seemed odd. The other problem was that the cosmic microwave background had nearly uniform temperature in every direction. This would not be much of a problem if the whole surface of matter-radiation decoupling had been in causal contact at some point after the Big Bang (i.e. light emitted at one point on the surface would have had time to reach any other point). Otherwise, this uniformity would be very bizarre. However the Big Bang theory at the time predicted that only regions of a few arcseconds across could have been in causal contact.

Both these problems were solved by Alan Guth in 1980. He proposed an addition to the history of the Universe. At the very early stage, for only a few miniscule fractions of a second, the Universe would undergo a period of exponential expansion, rather than the approximately linear one happening today. This period was called inflation and during it the Universe would expand by some 60 orders of magnitude. This is a factor of one with 60 zeroes to the right. Yes, it’s huge. This would allow the entire Universe to get into thermal contact and reduce curvature from any initial value to something so close to zero that there would be no noticeable deviation until now.

As yet another problem of the theory was solved, it was time to undertake more daring experiments. The exact nature of cosmic expansion was a problem for quite some time: Einstein’s field equations predicted a “cosmological constant” which would cause an accelerated expansion. However both Einstein himself and many other scientists thought of that result as some peculiarity in the mathematics, which did not have any physical significance. This question was put to test in 1997, when several projects (the Supernova cosmology project, led by Bruno Leibundgut, and the High-redshift Supernova Search Team, led by Brian Schmidt) almost simultaneously information about tens of Type Ia supernovae at large redshifts (in this case, large means 0.1 – 1). These results were used to calculate the values of both the Hubble constant and the cosmic deceleration parameter. It turned out that the “deceleration” is negative, i.e. the Universe is expanding increasingly faster, and that some kind of “cosmological constant” is really present. This prompted another overhaul of the cosmological theory, producing what is known as the LCDM (Lambda – Cold Dark Matter) variation of the Big Bang model.

And so this is where we stand now. After the accelerating expansion was discovered, no groundbreaking changes to the models were made – most efforts concentrated on getting better results for the important parameters. This is going to continue in the future, with a plethora of scientific projects planned. Some have already started their work – the Planck orbital telescope will observe the cosmic microwave background radiation with unparalleled precision, while its companion Herschel will look at the sky in infrared wavelengths. The Swift telescope is catching gamma ray bursts and an occasional supernova or two, providing different insights into the early Universe. Other projects are underway – Gaia will search for exoplanets in the Milky Way, while the James Webb telescope will replace Hubble and become our optical “eyes” for observing the Universe. One of them may find something that will once again change our view of what the Universe is and how it works. For all we know, this may be changed by some results obtained in a laboratory. Whatever the case, the future looks promising.

Thank you for reading all of this. As usual, comments are appreciated.