Abraham Loeb on universe expansion, new physics and way forward
The Big Bang Theory is the most widely regarded explanation for the the origin and evolution of the Universe. But recent developments in astronomy have exposed some discrepancies in the conclusions drawn from the theory and created a need for either new modifications in the theory or completely new physics to explain them.
For instance, in April 2019 Adam Riess and his colleagues at the Johns Hopkins University in the United States came up with the surprising conclusion that the Universe is expanding at a rate much faster than previously known. Scientists have measured the rate of expansion of the Universe — the Hubble Constant — in two different ways.
First: By observing the behaviour of far-off stars and galaxies in the present time and calculating the rate at which they are moving away from one another.
This has become possible only with modern telescopes, located both on Earth and in space — like the Hubble Space Telescope launched in 1990. These telescopes help astronomers in making direct observations of stars, their motion and the radiation (including light) coming from them.
The second way is to study the characteristics of cosmic microwave background (CMB) and other radiation that emanated at or after the birth of the Universe and extrapolating the trajectory to present time.
The measurements of CMB were made from the observations of the Planck Satellite operated by the European Space Agency from 2009 to 2013.
Riess’s research used the first method and concluded that the Universe was expanding at a rate of 73.4 kilometres per second per mega parsec (Km/sec/Mpc). That is 10 per cent faster than the 67.4 Km/sec/Mpc determined by using the CMB measurements.
The study also reduced the chances of this difference in rates resulting from an error to one-in-1,00,000 from one-in-3,000, making it almost certainly true.
Down to Earth spoke to Abraham Loeb, chair of the department of astronomy at Harvard University and the founding director of Harvard's Black Hole Initiative about the Big Bang Theory and its challenges.
Akshit Sangomla: The most widely accepted theory for the beginning of the Universe is the Big Bang Theory. How sure are scientists about this and why?
Abraham Loeb: The Big Bang model is supported by a large body of evidence. It postulates that the Universe started from a hot dense state, and we have detected the relic radiation left over from that state.
The model also assumes that the initial state was nearly uniform with small inhomogeneities that grew over time due to the attractive force of gravity to make the structure we see today in the form of galaxies and stars. The Milky Way, hosting our Sun, is an example.
Indeed, we find CMB to have almost exactly the same brightness in all directions on the sky with small variations of the appropriate magnitude, reflecting the expected initial state.
The model also predicts the abundances of light elements, like helium, deuterium and lithium, which were cooked in the first few minutes of the cosmic expansion after the Big Bang — when the Universe as a whole was hotter than the interior of stars.
The predicted abundances agree with observations. Altogether, the data we have provides robust support to the Big Bang model, but it also leads to several intriguing questions:
- What led to the Big Bang? In other words, what preceded the cosmic expansion we see?
In the very first instants, quantum mechanics was as important as gravity, but we still do not have a theory that unifies these two pillars of modern physics. So we cannot predict what may have happened before the Big Bang.
- We infer from cosmological data that most of the matter and energy in the present-day Universe are dark. We call them ‘dark matter’ and ‘dark energy’, but these are just labels that signify our ignorance. We would like to know.
AS: The expansion of the universe is accelerating and scientists do not seem to agree on the rate of acceleration. Why?
AL: All observers agree that the expansion of the Universe is accelerating due to dark energy, whose nature we do not understand but which represents the energy of the vacuum without matter.
But there is a debate regarding the expansion rate. We can infer the expansion rate that the Universe should have today based on the data we have on the CMB.
But some observers who measure the actual expansion rate today argue that their measured value disagrees with the expected value at a statistically significant level.
AS: Some scientists are calling for new physics to explain the discrepancies. What could this new physics be?
AL: The discrepancy in expansion rates can indicate some new physics or some unexpected behaviour of the dark matter and dark energy between the time when the CMB was produced (400,000 years after the Big Bang) and today. We do not know if that is the case.
AS: Dark energy and dark matter are the prominent unknowns in the current knowledge of the cosmos. What necessitated them and how close or far are we from understanding their true natures?
AL: We have evidence that there is much more matter out there than the ordinary matter we are made of.
First, the inhomogeneities at early times would have been smoothed out by the radiation if there was only ordinary matter. There needs to be a form of matter that does not couple to the radiation in order for galaxies like the Milky Way to form.
Second, when we look at galaxies, we infer that they must contain much more mass than the visible mass of their gas and stars. This was known for 70 years, since Fritz Zwicky inferred that clusters of galaxies contain much more matter than their visible mass.
But we still have no clue as to the nature of that dark matter. It is most likely made of particles that do not couple to light — this being the reason that we cannot see them — but we do not know what particles these are.
We know that the dark matter is unlikely to be made of primordial black holes because they would produce other effects that are ruled out observationally.
The breakthrough in our understanding could come from laboratory experiments. There were hopes that new particles will be produced and discovered at the Large Hadron Collider or other experiments, but so far we have had no success.
There were also attempts to detect dark matter particles impinging on Earth from the Milky Way galaxy, but again these “direct detection” experiments were not successful over the past few decades.
AS: What will the near and far future of the universe look like a) with humans in it and b) without humans in it?
AL: Without humans - in a few billion years, the Milky Way will collide with its nearest neighbour, the Andromeda galaxy and the night sky will change.
In about seven billion years, the Sun will die. At first it will expand to a red giant and possibly engulf the Earth and then its core will cool and contract to make a ‘white dwarf’, a piece of dense metal the size of the Earth.
Most of the stars have a mass that is ten times lower than that of the Sun and they will continue to shine for up to 10 trillion years — a thousand times longer than the Sun. After that, there will be darkness.
If the accelerated expansion of the universe will continue, it will become a dark and lonely place with our galaxy — the product of the merger of the Milky Way and Andromeda — surrounded by a vacuum.
With humans - since technology evolves exponentially with a time constant of a few years, we will witness vast advances in artificial intelligence (AI), robotics, and genetics. Within a thousand years, humans will build machines that transcend them and can venture into a long journey into space.
Three-dimensional printers, equipped with AI, will produce life as we know it on other planets out of the raw materials there. We might also find evidence for other civilizations that are far more advanced than we are.
After reading the newspaper every day, I have serious doubts that we are the “smartest kid on the black.”