For centuries, we have arrogantly believed that we have found almost all the answers to our deepest questions. Scientists thought Newtonian mechanics described everything until they discovered the wave nature of light. Physicists thought that when Maxwell unified electromagnetism it was the finish line, but then relativity and quantum mechanics came along. Many thought the nature of matter was completely clear when we found the proton, neutron and electron, but then we stumbled upon high-energy particles. In just 25 years, five incredible discoveries have reshaped our understanding of the universe, and each one promises an epic revolution. We live in an amazing time: we have the opportunity to look into the very depths of the mysteries of all things.
Neutrino mass
When we started counting on paper the neutrinos that come from the Sun, we got a number based on the fusion that must be taking place inside. But when we actually started counting neutrinos coming from the Sun, we saw only a third of what was expected. Why? The answer only emerged recently when a combination of measurements of solar and atmospheric neutrinos showed that they could oscillate from one type to another. Because they have mass.
What does this mean for astrophysics. Neutrinos are the most abundant massive particles in the Universe: there are a billion times more than electrons. If they have mass, it follows that:
- they make up a fraction of dark matter, - fall into galactic structures, Promotional video:
- possibly form a strange astrophysical state known as fermion condensate,
- may be associated with dark energy.
If neutrinos have mass, they could also be Majorana particles (rather than the more common Dirac-type particles), providing a new type of nuclear decay. They may also have super-heavy left-handed brethren that could explain dark matter. Neutrinos also carry most of the energy in supernovae, are responsible for cooling neutron stars, affect the afterglow of the Big Bang (CMB), and are an essential part of modern cosmology and astrophysics.
Accelerating Universe
If the Universe begins with a hot Big Bang, it will have two important properties: an initial expansion rate and an initial matter / radiation / energy density. If the density were too great, the universe would be reunited again; if too small, the universe would expand forever. But in our Universe, density and expansion are not only perfectly balanced, but a tiny fraction of this energy comes in the form of dark energy, which means that our Universe began to expand rapidly after 8 billion years and has continued in the same spirit since then.
What does this mean for astrophysics. For the first time in the history of mankind, we got the opportunity to learn a little about the fate of the universe. All objects that are not gravitationally connected with each other will eventually scatter, which means that everything outside our local group will one day fly away. But what is the nature of dark energy? Is this really a cosmological constant? Is it related to the quantum vacuum? Could it be a field whose strength changes over time? Future missions like ESA's Euclid, NASA's WFIRST and new 30-meter telescopes will allow more accurate measurements of dark energy and allow us to accurately characterize how the universe is accelerating. After all, if the acceleration increases, the Universe will end in a Big Rip; if it falls, with a Big Compression. The fate of the entire universe is at stake.
Exoplanets
A generation ago, we thought there were planets near other star systems, but we had no evidence to support this thesis. Currently, thanks in large part to the NASA Kepler mission, we have found and tested thousands of these. Many solar systems are different from ours: some contain super-Earths or mini-Neptunes; some contain gas giants in the interior of solar systems; most contain worlds the size of Earth at the correct distance from tiny, faint, red dwarf stars for liquid water to exist on the surface. Yet much remains to be seen.
What does this mean for astrophysics. For the first time in history, we have discovered worlds that could be potential candidates for life. We are closer than ever before to detecting signs of alien life in the universe. And many of these worlds may someday be home to human colonies if we choose to take this path. In the 21st century, we will begin to explore these possibilities: measure the atmospheres of these worlds and look for signs of life, send space probes at a significant speed, analyze them for similarity to the Earth in terms of such features as oceans and continents, cloud cover, oxygen content in the atmosphere, times of the year. Never in the history of the universe has there been a more suitable moment for this.
Higgs boson
The discovery of the Higgs particle in the early 2010s finally completed the Standard Model of elementary particles. The Higgs boson has a mass of about 126 GeV / s2, decays after 10-24 seconds and decays exactly as predicted by the Standard Model. There is no sign of new physics outside of the Standard Model in the behavior of this particle, and that's a big problem.
What does this mean for astrophysics. Why is the Higgs mass much less than the Planck mass? This question can be formulated in different ways: why is the gravitational force so weaker than the other forces? There are many possible solutions: supersymmetry, extra dimensions, fundamental excitations (conformal solution), Higgs as a constituent particle (technicolor), etc. But so far these solutions have no proofs, and have we looked carefully enough?
At some level, there must be something fundamentally new: new particles, new fields, new forces, etc. All of them by their nature will have astrophysical and cosmological consequences, and all these effects depend on the model. If particle physics, for example at the LHC, does not provide any new hints, perhaps astrophysics will. What happens at the highest energies and at the shortest distances? The Big Bang - and cosmic rays - brought us the highest energies than our most powerful particle accelerator could ever have. The next key to solving one of the biggest problems in physics may come from space, not on Earth.
Gravitational waves
For 101 years, this has been the holy grail of astrophysics: the search for direct evidence of Einstein's biggest unproven prediction. When Advanced LIGO went online in 2015, it was able to achieve the sensitivity needed to detect ripples in spacetime from the shortest wavelength source of gravitational waves in the Universe: coiling and merging black holes. With two confirmed detections under its belt (and how many more will be), Advanced LIGO has taken gravitational wave astronomy from fantasy to reality.
What does this mean for astrophysics. All astronomy until now has been dependent on light, from gamma rays to the visible spectrum, microwave and radio frequencies. But detecting ripples in spacetime is an entirely new way of studying astrophysical phenomena in the universe. With the right detectors with the right sensitivity, we can see:
- merging neutron stars (and find out if they create gamma ray bursts);
- the merger of white dwarfs (and we associate type Ia supernovae with them);
- supermassive black holes devouring other masses;
- gravitational-wave signatures of supernovae;
- signatures of pulsars;
- residual gravitational-wave signatures of the birth of the Universe, possibly.
Now gravitational wave astronomy is at the very start of development, hardly becoming a proven field. The next steps will be to increase the range of sensitivity and frequencies, as well as the comparison of what is seen in the gravitational sky with the optical sky. The future is coming.
And we are not talking about other great puzzles. There is dark matter: more than 80% of the mass of the Universe is completely invisible to light and ordinary (atomic) matter. There is the problem of baryogenesis: why is our universe full of matter and not antimatter, even though every reaction we have ever observed is completely symmetrical in matter and antimatter. There are paradoxes of black holes, cosmic inflation, and a successful quantum theory of gravity has not yet been created.
There is always a temptation to believe that our best days are behind us, and that the most important and revolutionary discoveries have already been made. But if we want to comprehend the biggest questions of all - where did the Universe come from, what it actually consists of, how it appeared and where it is going, how it will end - we still have a lot of work to do. With telescopes unprecedented in size, range and sensitivity, we can learn more than we ever knew. Victory is never guaranteed, but every step we take brings us one step closer to our destination. It doesn't matter where this journey takes us, the main thing is that it will be incredible.