The expansion of the universe did not always have the same speed
The universe is expanding, that is, all objects that are not held by gravity move away from each other. The speed of the expansion of the universe is however subject to debate. This is one of the big questions of cosmology. In the commonly accepted model, the speed of expansion of the universe has been variable over time.
At the beginning of its existence, the universe would have undergone a phase of extreme inflation that would have allowed it to grow very quickly. Then it slowed down at a slower pace because of the gravitational effect of dark matter. This phase would have lasted a few hundred thousand years. Since that time, the expansion of the universe would accelerate again, this time under the influence of dark energy.
The Hubble Constant, a standard for measuring the speed of expansion of the universe
This story is partly due to measurements of the Hubble Constant, an indicator that describes the rate of expansion of the universe at a given moment. The problem is that depending on the scientific instruments and methods used, the measurements of the Hubble Constant do not always agree.
Some astrophysicists rely on observations of the cosmic microwave background (CMB), the fossil of the first light of the universe emitted when the universe was 380,000 years old. This allows them to estimate that the Hubble Constant is 67 km/s per megaparsec. This means that a galaxy at 1 megaparsec, about 6.5 million light-years, is moving away from us at 67 km/s while a galaxy at 2 megaparsecs is moving twice as fast.
Study shows expansion of universe is faster than we thought
When one wants to make measurements on real galaxies more or less distant, one obtains a Hubble Constant equal to 74 km/s by megaparsec, and not 67 km/s. The more years pass, the more difficult it is to reconcile these two figures. Witness a new study published by a team of astrophysicists from the University of California. Thanks to the Hubble space telescope and the Keck observatory, they used a method that uses gravitational lenses.
What is interesting is that to avoid any bias they conducted a blind study, that is to say, they hid themselves the result until they were sure of having eliminated all sources of errors. Like all of us, astronomers may be subject to cognitive biases. They can unconsciously adjust a set of data to match a cosmological model. But despite these extra precautions, their result does not solve the dilemma. Like all measurements made locally, they get a Hubble Constant of 75 km/s per megaparsec.
What is the true speed of the expansion of the universe ?
Between 67 km/s and 75 km/s, the difference is too big to be considered as a margin of error. Astrophysicists are therefore faced with a dilemma. Either there is a problem with the measurement of the rate of expansion of the universe acquired thanks to the cosmic microwave background, which most astrophysicists consider improbable, or it is necessary to review the standard model of cosmology.
For example, the properties of dark energy may have changed over time, or our observations are too imprecise. Let’s hope that the arrival of bigger observatories and ever more efficient methods will solve this puzzle. But if astronomers are not able to reconcile these two measures, it may be necessary to revisit in depth our history of the universe.
Expansion of the universe, dark energy : the challenges of DESI
Dark energy is one of the toughest puzzles in modern physics. For twenty years, a series of measures seem to show that the expansion of the universe accelerates over time. Since none of the four fundamental forces allowed by physics can explain this phenomenon, so far the phenomenon responsible for this acceleration is called black energy.
If we do not understand dark energy, we can at least try to measure its effects. That’s why DESI was created. This scientific instrument will work from the Mayall telescope and its 4 meter mirror. DESI will observe tens of millions of quasars and galaxies, which is expected to create a 3D map of the universe spanning up to 11 billion light-years.
This map of unprecedented size and precision should allow us to grasp a little better how the large structures of the universe are distributed, evolved and the role played by dark energy. This will be an opportunity to test complements or alternatives to general relativity. DESI will accurately measure distances based on the traces left by so-called acoustic oscillations of the baryons, that is to say the prints left by the acoustic waves, the sound, in the plasma of the universe. This is the same method that will be used by the Euclid European Space Observatory from the L2 Lagrange point of the Sun-Earth system, from 2022.
DESI’s tests should begin quickly. The teams working on the project hope to recover complete data by 2025, which may provide a better understanding of what is called dark energy.
Expansion of the universe could be measured through quasars
– News of February 5, 2019 –
Dark energy is one of the most important mysteries of modern astronomy. We know since 20 years that the expansion of the universe is accelerating. Our universe is growing every day and growing faster and faster. If the universe consisted entirely of mass and obeyed on a large scale only to gravity, it could only slow down. But it’s the opposite that we seem to observe.
Several explanations for this phenomenon could be formulated. Maybe our measures are wrong. However, more and more of them are making the same conclusions. Perhaps our understanding of gravity on a large scale is not yet complete. Or we can imagine that an energy of unknown nature pushes the universe to grow. It may be a kind of fundamental constant that is simply part of the laws of nature.
The big problem when trying to determine the speed of expansion of the universe is that we do not know how to measure large distances very well. This can be done fairly efficiently for recent periods by observing the brightness of type 1A supernovas. We can also probe the beginnings of the history of the universe thanks to the cosmic microwave background (CMB) that gives us a good picture of what the universe was 380000 years after the big bang. But between these two extremes, it is difficult to evaluate the Hubble constant, the magnitude that gives the rate of expansion of the universe at a given moment.
An Italian team thinks they have found a new way to determine the distance and therefore to measure the speed of expansion of the universe. This involves observing the luminosity of the quasars, these extremely bright galaxy nuclei which are therefore observable from very far away. Unlike type 1A supernovas, not all quasars have the same absolute magnitude. When we observe a quasar less luminous than another, we do not know if it is because it is farther away or if it is at the same distance but less energetic.
To overcome this obstacle, the Italian team compared the light of quasars in two different spectral bands, X-ray and ultraviolet. By establishing a ratio between these two luminous fluxes, they found constants which make it possible to determine the distances. Nearly 1600 quasars were studied with this method, which allowed us to write a somewhat more complete history of the expansion of the universe.
The first results of this study suggest that dark energy is gaining intensity over time. So not only is the expansion of the universe accelerating, but the element that causes it is itself constantly evolving. If this result is confirmed, which could take years, then it will be necessary to review numerous models which try to explain the dark energy. Dark energy would have an increasingly important role in the universe, constantly moving galaxies and stars away. At a sufficiently large time scale, even the particles that make up the atoms would be far apart, which would probably mean the end of time.
The expansion of the universe would be faster and faster
– News of February 27, 2018 –
The Hubble space telescope has been operational for almost 30 years and continues to deliver very important results. Recent observations have shown that the acceleration of the expansion of the universe is actually much faster than expected. The problem comes from a comparison of Hubble data with those of the Planck space observatory obtained a few years earlier. The measurements of both instruments are considered very reliable. They both sought to determine the Hubble constant, a parameter that describes the rate of expansion of the universe at a given moment. Planck’s data placed this constant between 67 km and 69 km per second per megaparsec. Planck had made these measurements by studying the cosmic microwave background, the very first light of the 13.8 billion-year-old universe.
New Hubble data estimates the Hubble constant at 63 km per second per megaparsec. Hubble has observed Cepheids, which are variable stars much more recent. Between the two measures, there is 9% difference. The problem is that there are difficulties in explaining it. The probability that an error has slipped into one of the two measures is incredibly low. These results seem to show that the expansion of the universe has accelerated in its history, and even faster than originally thought.
Among the possible explanations there is of course the dark energy. Failing to be well defined, it is a parameter that can be added to the standard model of cosmology to explain this type of observation. Another possibility is stronger interaction than expected between dark matter and more conventional materials. Finally new models continue to emerge to try to provide an explanation for this phenomenon, for example black radiation, sterile neutrinos that would be affected only by gravity.
The earliest evidence of accelerating universe expansion is very recent, dating back to the late 1990s, and it is likely that more decades of observation and speculation will be needed before consensus can be reached about the mechanism causing this acceleration. This is why we regret the very likely cancellation of WFIRST because the instrument would have allowed measurements of the Hubble constant at different ages of the universe. Fortunately, Europe has not canceled Euclid, its future space observatory dedicated to research on the expansion of the universe and dark energy. It should come into service at the beginning of the next decade. It will try to go back up to 10 billion years in the past. The different models of dark energy present tiny variations, so it will take measurements of very high precision to determine which track is the most interesting to follow.
Image by NASA / WMAP Science Team [Public domain], via Wikimedia Commons
