When we observe the sky at night, we peer up at a sea of incredible blackness, swimming with a myriad of dazzling dabs of distant starlight. Much more is unknown about the Universe than is known–and, it has been said, the Universe is not only weirder than we imagine, it is weirder than we can imagine. What we think we know is this: the Universe was born about 13.8 billion years ago in the faster-than-the-speed-of-light exponential inflation of Space and Time at the instant of the Big Bang–ballooning wildly from the size of an elementary particle to attain macroscopic size in the merest fraction of a second. The Universe keeps its secrets well, and to understand this incomprehensibly immense and strange swath of Spacetime, of which we are a part, we seek to find the elusive answer to the most profound and important of all mysteries–the mystery of the origin of all that we are and all that we will ever know. In January 2017, an international team of astronomers announced that, by using distant galaxies as gravitational lenses, they had made new measurements supporting the observation that the Universe is currently expanding faster than expected.
The astronomers, using the NASA/European Space Agency (ESA) Hubble Space Telescope (HST), found that the expansion rate for the local Universe is consistent with earlier studies that used different methods. Nevertheless, there are differences that suggest something intriguing is missing and waiting to be discovered. This scientific sirens’ song seduces curious astronomers because it suggests that there may be a fundamental problem buried deep within the very heart of their scientific understanding of the Universe.
One of the most important Cosmological discoveries ever made is that the Universe is expanding. This discovery forced cosmologists to consider dynamic models of the Universe, rather than viewing it as both static and eternal, as had previously been the case. Furthermore, this finding also suggests the existence of a timescale or age for the Universe–that it was born about 13.8 billion years ago in the wild inflation of the Big Bang. The great astronomer Edwin Hubble, in whose honor the HST is named, deserves most of the credit for the discovery of the expansion. The expansion rate of the Universe also bears his name, and is now called the Hubble Constant.
It is generally thought that our Universe started out as an unimaginably small Patch that was almost–but not precisely–nothing. It is still unknown exactly what caused that extremely small Patch to balloon in size and then evolve into the Cosmic Wonderland that we observe today. Everything we are, everything we know, originated from that mysterious Patch that started out smaller than a proton.
The baby Universe was filled with a violent, stormy sea of energetic radiation–a strange soup of searing-hot particles of light (photons). So, here we are now, almost 14 billion years later, standing helplessly on our small and rocky little planet, watching hopelessly as the furious fires of our cooling and expanding Universe fade away–going out like a bright little candle lost in eternity.
The currently favored model proposes that, at the very instant of its birth, the baby Universe experienced an exquisitely brief era of inflation. The most recent measurements indicate that inflation is the most likely explanation known that could have caused the Universe to develop the way that it has. As incredible as it may seem, inflation blew up–like a bizarre bubble–every region of the small Patch of Space by a factor of at least 10 to the 27th power (10 followed by 26 zeroes). Before inflation created this weird “bubble”, the region of the Universe that we are able to see today–the observable Universe–was an elementary-particle-sized smooth entity.
During our Universe’s babyhood, it was composed of a weird plasma of elementary particles. Energetic, speedy high-energy photons slowly lost their energy over time. As a result, these photons began to travel more lazily through Space. This basically means that the photons cooled off as the Universe continued to expand at a much more stately pace than it had during the brief era of inflation. The energy that had been churned out flowed into the expansion, and in the 13.8 billion years since our Universe was born, it has expanded by yet another 10 to the 27th power.
A mysterious and indisputably weird substance, called the dark energy, is thought to be the secretive, well-hidden culprit behind the Universe’s currently accelerating rate of expansion. What is the dark energy? The truth is that we do not know. However, one generally accepted model suggests that because Space is literally everywhere, the dark energy must also be everywhere. This is because this dark stuff seems to be a property of Space itself. The effects of the dark energy become increasingly more and more powerful as Space expands. However, the force of gravity becomes increasingly more and more powerful when objects are close together–and weaker when they are farther apart. Because the force of gravity becomes weaker as Space expands–pulling objects farther and farther apart–dark energy now accounts for over two-thirds of the energy of the Universe. This indicates that approximately 74% of the Universe has not been identified.
This sort of mystery tends to captivate scientific detectives. This is because such an important and unanswered question shows that there is a bewitching gap in our scientific understanding that needs to be explained. This, of course, is very seductive because it hints intriguingly that there is new physics waiting to be discovered. This indicates that the Universe may be very different from what has earlier been proposed.
Physicists realize that radiation (waves of light) carry energy. Albert Einstein’s well-known equation E=mc squared says that matter and energy are really the same–and, as such, are interchangeable. For example, all stars–including our Sun–are powered by the conversion of mass into energy.
But energy must have a source–either matter or radiation. The idea here is that Space, even when it appears to be devoid of matter and radiation, nevertheless contains its own residual supply of energy–the energy of the vacuum. This vacuum energy, when applied to the Cosmos itself, creates a force that accelerates the expansion of the Universe.
In June 2016, a team of astronomers announced that they had used the HST to measure the distances to remote stars dwelling within nineteen galaxies. The astronomers discovered, to their amazement, that the Universe is currently expanding much faster than the rate expected.
The confirmation of this discovery provides an important clue pertaining to our scientific understanding of three of the Universe’s most mysterious and elusive components: dark energy, dark matter, and neutrinos. The dark matter, which is thought to be composed of some exotic form of non-atomic matter, is much more abundant than so-called “ordinary” atomic matter. The most recent measurements of the composition of our Universe suggest that 68% of it is dark energy, 27% of it is dark matter, and a mere 5% of it is “ordinary” atomic matter–the runt of the Cosmic litter of three. We are familiar with “ordinary” atomic matter–but we are in the dark about dark energy and dark matter.
The team of astronomers was led by Nobel Laureate, Dr. Adam Riess, who is of Johns Hopkins University and the Space Telescope Science Institute (STSI) in Baltimore, Maryland. Dr. Riess was awarded the 2011 Nobel Prize in Physics for being one of the first astronomers to discover that the Universe is expanding faster than expected, under the influence of the previously unknown dark energy. In the 2016 study, Dr. Riess and his team again used HST to make their observation that suggests the Universe is expanding between five and nine percent faster than previously calculated from their original observation. This more recent measurement conflicts with the rate predicted from earlier measurements of the baby Cosmos. The team measured the Hubble Constant to an unprecedented accuracy, thus reducing the uncertainty to a mere 2.4 percent.
The refined calculation of the Hubble Constant was made by Dr. Riess’s team using very precise measurements of distances to both remote and nearby galaxies using HST. These improved distance measurements were conducted by streamlining and strengthening the “cosmic distance ladder”, which astronomers use in order to measure distances to galaxies accurately. Dr. Riess’s team compared these measured distances to the expansion of the Universe as measured by the stretching of light emitted from galaxies that are traveling farther and farther away from us (redshift). The astronomers then used these two values in order to calculate the new Hubble Constant.
Cosmic Lenses Shed Strange New Light On The Universe’s Mysteries
The Hubble Constant, the rate at which the Universe is expanding, is one of the fundamental quantities describing our Universe. The international group of astronomers, that released their findings in January 2017, came from the HOLiCOW collaboration, led by Dr. Sherry Suyu, associated with the Max Planck Institute for Astrophysics in Germany, the ASIAA in Taiwan and the Technical University of Munich, Germany. This international team used HST, as well as other telescopes in space and on the ground to observe a quintet of galaxies in order to calculate an independent measurement of the Hubble Constant.
This new measurement was made completely independent of (yet it is in excellent agreement with) other measurements of the Hubble Constant in the local Universe that used Cepheid Variable stars and supernovae as points of reference. A Cepheid Variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. Named after delta-Cephei, Cepheid Variables are the most important type of variable because it has been found that their periods of variability are related to their absolute luminosity. This makes them extremely valuable as contributors to astronomical distance measurements.
For short distances in Space, that are within our own Milky Way Galaxy or within our Galaxy’s Local Group of nearby galaxies, astronomers use Cepheid Variables as standard candles. However, beyond the Local Group of nearby galaxies, at much greater distances, telescopes are unable to resolve individual stars. Astronomers, observing celestial objects that are very far away, can only observe the collective light of large groups of stars. Therefore, in order to measure distances to remote galaxies, astronomers need to find bright objects–very, very bright objects. So, astronomers turn to exploding stars that have come to the end of the stellar road. These exploding stars, called supernovae, blast themselves to pieces within their host galaxy approximately every 100 years. Supernovae are among the brightest events in the sky. Indeed, astronomers can sometimes spot a supernova even when they can’t see its parent galaxy.
In order to measure these much greater distances, astronomers turn to a certain special type of exploding star termed a Type Ia supernova. Type Ia supernovae explode in a binary system where a duo of sister stars, in orbit around one another, experience something rather ghastly. One of the stars in this doomed system must be a white dwarf, which is the very dense, carbon composed relic of a small star that, during its hydrogen-burning “life” on the Hertzsprung-Russell Diagram of Stellar Evolution, was about the same size as our Sun. The other can be a giant star or an even smaller white dwarf. Alas, the sinister white dwarf that inhabits this very unfortunate system has vampire-like cravings, and continually sips up the stellar matter of its unfortunate companion star. However, this terrible feast eventually backfires on the white dwarf. At some point, this stellar relic drinks up more of its sister’s material than it should, and it reaches critical mass–exploding as a Type Ia supernova. It is generally thought that Type Ia supernovae all show the same level of brightness. For this reason, they make excellent standard candles that astronomers can use for distance measurements–including the accelerating rate of the universal expansion, that revealed the existence of the dark energy.
A problem arose because the value of the Hubble Constant, measured by Dr. Suyu and her colleagues, as well as those measured by astronomers using Cepheids and supernovae, are different from the measurement made by the European Space Agency’s (ESA’s) Planck Satellite. However, there is an important difference between the studies. This is because Planck measured the Hubble Constant for the early Universe by observing the Cosmic Microwave Background (CMB) radiation. The CMB is the relic radiation left over from the Big Bang itself, stretched out to microwave wavelengths as a result of the expansion of the Universe–and, as such, it reveals the primordial anisotropies of the Big Bang fireball amplified to cosmological proportions.
Even though the value of the Hubble Constant determined by Planck fits well with the current scientific understanding of the Universe, the values obtained by the different teams of astronomers for the local Universe do not agree with the accepted theoretical model of the Universe. “The expansion rate of the Universe is now starting to be measured in different ways with such high precision that actual discrepancies may possibly point towards new physics beyond our current knowledge of the Universe,” Dr. Suyu explained in the January 26, 2017 STSI Press Release.
The targets of Dr. Suyu’s study were massive galaxies situated between Earth and extremely remote glaring quasars–which are brilliant galaxy cores powered by the fiery accretion disks surrounding super massive black holes in the early Cosmos. The light traveling from the more distant quasars is bent around the enormous masses of the foreground galaxies as a result of strong gravitational lensing. This creates multiple images of the background quasar, some smeared out to create extended arcs.
However, galaxies do not form perfectly spherical distortions in the fabric of Spacetime, and the lensing galaxies and quasars are not precisely aligned. Because of this, the light flowing from the different images of the background quasar follows paths which have slightly different lengths. Because the brightness of quasars changes as time goes by, astronomers can observe the different images flicker at different times. This is because of the delays between them that depend on the lengths of the paths the light has followed. These delays are directly related to the value of the Hubble Constant.
By using precise measurements of the time delays between the multiple, differing images, as well as by using supercomputer models, the astronomers were able to measure the Hubble Constant to an impressively high precision: 3.8%.
Team member Dr. Vivien Bonvin, from the Laboratory of Astrophysics, EPFL, in Switzerland, noted in the STSI Press Release: “An accurate measurement of the Hubble Constant is one of the most sought-after prizes in cosmological research today.”
Dr. Suyu added in the same Press Release that “The Hubble Constant is crucial for modern astronomy as it can help to confirm or refute whether our picture of the Universe–composed of dark energy, dark matter, and normal matter–is actually correct, or if we are missing something fundamental.”