News from the Frontiers of Cosmology: A companion to the book The Edge of Physics
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Why astronomy matters

During the writing of The Edge of Physics, I was struck by the role of astronomy in changing fundamental perceptions about our universe, ourselves and our place in the universe. In fact, each such epochal moment can be traced to a unique (set of) astronomical observation(s).

The Copernican Revolution, which helped fuel the Scientific Revolution, caused us to change our entire world view. Earth was no longer the centre of the solar system. Instead, Copernicus argued that the motion of planets was best explained if the planets revolved around the sun.

Fast forward to the early 1900s, when our entire universe was the Milky Way. Even Einstein thought so, which led to what he called his greatest blunder. When his own equations of general relativity showed that the universe had to be either contracting or expanding, Einstein introduced a “cosmological constant”—a fix that made the universe static. Then, Edwin Hubble and Milton Humason made a series of observations using the 100-inch telescope atop Mount Wilson which showed that the universe consisted of much more than the Milky Way. More importantly, almost all of these galaxies were moving away from us (astronomer Vesto Melvin Slipher should be credited with measuring the redshift of these galaxies before Hubble, but it was Hubble who showed that these galaxies lay beyond the confines of our galaxy).

With Hubble’s work (and the work of theoreticians), the idea of a Big Bang began to take hold. Our universe, it seemed, had begun in a fireball. But the theory had to share the stage with the steady state model of the universe. The Big Bang model was confirmed in the 1960s with the accidental yet monumental detection of the cosmic microwave background (CMB) – a radiation leftover from the hot early universe. While many, many more experiments had to be done to nail down the properties of the CMB, we think of the 1965 discovery by Penzias and Wilson as the beginning of the big bang-era.

Also in the 1960s came the discovery of dark matter – based on observations of how fast the stars and gas are moving in the Andromeda Galaxy. With Vera Rubin’s pioneering astronomy, our universe went from being made of matter to one in which nearly 90 per cent of the matter was made of unknown stuff called dark matter.

Nothing dramatic happened for a few decades. Then in the late 1990s came the discovery of dark energy. Again, it was a set of astronomical observations of supernovae that showed that the expansion of the universe was accelerating. The favoured explanation is this: the fabric of spacetime has inherent energy, which is causing the accelerated expansion. Significantly, our understanding of the composition of the universe changed beyond belief. We now think it is made of mostly mysterious stuff: 73 % dark energy, 23 % dark matter, and only 4 % normal matter.

Hidden in the discovery of dark energy is the potential for a revolution just as (if not more) significant than the Copernican Revolution. Physicists are struggling to explain why dark energy has the value it does. Nothing in known physics can help. It’s value seems to be just right for galaxies, stars, planets and hence life to form, and produce physicists who are asking the question: why is our universe the way it is? One idea – controversial, but by no means lacking support among big-name physicists – is that our universe is part of a multiverse, which is the name cosmologists give to an ensemble of universes (could even be an infinity of universes). In a multiverse, dark energy takes a random value in each universe, and we just happen to be living in which its value is conducive to the emergence of life.

How on earth do we ever detect other universes? Well, if we had a theory that predicted a multiverse, and we verified many, many elements of that theory in our universe, then we’d have to – however reluctantly – accept the existence of a multiverse. Here’s Steven Weinberg (quoted in The Edge of Physics): “The important thing is not whether you can observe every ingredient in a theory, the important thing is whether you can observe enough of the consequences of the theory to test it and confirm that the theory is right.”

If the existence of a multiverse is somehow verified by experiments, then the discovery of dark energy may become the astronomical observation that we will look back on as the one that begat a multiverse.

The Edge of Physics


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