How did cosmology become science
The most important astronomical discoveries at a glance
One event was of particular astrophysical interest because it indicated the merger of two neutron stars. In contrast to the merging of black holes, such collisions between two ultra-dense stars create fireworks of optical light, X-rays and gamma rays. The discovery filled a void in the classic work of BBFH's work: the authors had explained the formation of many elements in space, but were confused in the case of gold. In the 1970s, David N. Schramm and his colleagues had speculated that the exotic nuclear processes involved in neutron star mergers might do the job - a theory that has since been confirmed.
The big open questions
Despite the incredible advances in astronomy over the past 175 years, we may have more questions today than in previous centuries. Take dark matter. I said more than 20 years ago that we would understand the nature of dark matter long before today. Although this prediction has been proven wrong, I haven't given up hope.
Dark energy is a different story: It came into play in 1998 when researchers measured the distances and speeds of supernovae and found that the universe's expansion was indeed accelerating. The gravitation pulling galaxies together seemed to be dominated by a mysterious new force latent in empty space that is causing the galaxies to drift apart - a force known as dark energy. The mystery of dark energy has persisted - we still don't know what causes it and why it has the particular strength that it possesses. And we probably won't understand them until we have a model of the graininess of space on a scale a billion billion times smaller than an atomic nucleus. Theorists working on string theory or loop quantum gravity face this challenge. However, the phenomenon seems so far removed from experimental testing that I do not expect any answers in the near future. The positive thing, however, is that a theory that can explain the energy in the vacuum of space could also provide insight into the beginnings of our universe, when everything was so compressed and dense that quantum fluctuations could shake the entire cosmos.
Which brings us to another big question that we now face: How did it all start? What exactly triggered the big bang that started our universe? Did space undergo a period of extremely rapid early expansion called inflation, as many theorists believe? And there is something else: some models, such as eternal inflation, suggest that “our” big bang could only be an island of spacetime in a huge archipelago - one big bang among many. If this hypothesis is true, different Big Bangs could cool differently, which in each case leads to unique physical laws - a "multiverse" instead of a universe. Some physicists hate the concept of the multiverse because it means we will never have reasonable explanations for the fundamental numbers that govern our laws of physics. From this broader perspective, they might just be "accidents." But our preferences are irrelevant to nature.
About ten years ago I was on a panel discussion at Stanford University where a listener asked us how much we would bet on the concept of the multiverse. I replied that on a scale where I was betting on my goldfish, my dog, or my life, I was close to dog-level. Andrei Linde, who had campaigned for perpetual inflation for 25 years, said he would almost bet his life. Later, when he was told this, physicist Steven Weinberg said he would be happy to bet for the lives of my dog and Lindes. Linde, my dog, and I will all be dead before the question is resolved. But none of this should be dismissed as metaphysics. It's speculative science - exciting science. And it may be true.
Prospect of the end
And what will happen to this universe - or our multiverse? Long-term predictions are seldom reliable, but the best and most conservative bet is that we will have almost an eternity with an ever colder and emptier cosmos ahead of us. Galaxies will move away and disappear faster and faster. All that will be left of our vantage point will be the remains of the Milky Way System, the Andromeda Galaxy, and smaller neighboring systems. Protons can decay, dark matter particles can be destroyed; there can be occasional flashes of light as black holes evaporate - and then silence follows. This possible future is based on the assumption that dark energy remains constant. If it crumbles, however, it could lead to a "big crunch" in which the universe contracts into itself. Or if the dark energy intensifies, there would be a "big rip" because galaxies, stars and even atoms are torn apart.
Other questions, closer to our cosmic homeland, torment us: Could there be life on one of these new planets that we are discovering? Here we are still in the realm of speculation. But unless the origin of life on earth was a rare coincidence, I expect evidence of a biosphere on an exoplanet within 20 years. I don't expect aliens to be discovered anytime soon, but I think finding extraterrestrial intelligence is a worthwhile venture. Success in the search would mean that the concepts of logic and physics are not limited to the "hardware" in human skulls. So far, 95 percent of the progress in cosmology and astrophysics has been due to the further development of instruments and technology and less than 5 percent to the "armchair theory". I assume that this balance will persist.
"Only when the empirical resources are exhausted do we have to go into the dreamy realms of speculation"
What Hubble wrote in the 1930s is still a good maxim today: "Only when the empirical resources are exhausted do we have to go into the dreamy realms of speculation." There have been many particularly exciting epochs in the past 175 years: the 1920s and 1930s when we realized that the universe is not limited to the Milky Way system; and the 1960s and 1970s when we discovered objects that defied classical physics, such as neutron stars and quasars, and when we found evidence of the beginning of time in the form of cosmic microwave background radiation.
Since then, the pace of progress has stabilized rather than slowed. When the history of science is written, we will celebrate this amazing advance as one of its greatest triumphs - with plate tectonics, the genome, and the Standard Model of particle physics. And some important areas of astronomy are just gaining momentum: Exoplanet research is only 25 years old, and serious work in astrobiology is still in its infancy. Some exoplanets might have life - they might even harbor aliens who already know all the answers. I find that encouraging.
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