AKSHIT SANGOMLA ABRAHAM LOEB,

All observers agree that the expansion of the universe is accelerating (owing to “dark energy”, whose nature we do not understand, but which represents the energy of 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 cosmic microwave background

[CMB is an elusive radiation that emanated at or after the birth of the universe and extrapolating its trajectory to present time is one of the ways to calculate the expansion of the universe]. 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.

Bang) and today. We do not know if that is the case. [Scientists estimate that 27 per cent of the universe is dark matter which does not absorb, emit or reflect light and whose existence is inferred only from the gravitational effect it seems to exert on visible matter.]

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 type 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.

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.

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, of which the Milky Way hosting our Sun are examples. Indeed, we find the cosmic microwave background to have almost exactly the same brightness in all directions in 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 intriguing questions:

(i) What led to the Big Bang? 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 and so we cannot predict what may have happened before the Big Bang.

(ii) 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.

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. 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 10 times lower than that of the Sun and they will continue to shine for up to 10 trillion years (1,000 times longer than the Sun). After that, there will be darkness. If the

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

accelerated expansion of the universe continues, it will become a dark and lonely place with our galaxy (the merger product of the Milky Way and Andromeda) surrounded by 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. 3D 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 civilisations that are far more advanced than we are.