During the inflationary phase of the early universe, quantum fluctuations in the vacuum generate particles as they stretch beyond the Hubble length. These fluctuations are thought to result in the density fluctuations and gravitational radiation that we can try to observe today. It is possible to calculate the quantum-mechanical evolution of these fluctuations during inflation and the subsequen...

There is a famous maxim in science known as Occam's Razor. Put simply, the axiom states that when considering competing theories to explain a phenomenon, one should choose the theory that is simplest with the fewest assumptions. And by implication, if evidence does not exist to substantiate a claim beyond a reasonable doubt, a negative conclusion should be drawn. This maxim is often evoked when...

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The origin of cosmic rays is one of the major unresolved astrophysical questions. In particular, the highest energy cosmic rays observed possess macroscopic energies and their origin is likely to be associated with the most energetic processes in the Universe. Their existence triggered a flurry of theoretical explanations ranging from conventional shock acceleration to particle physics beyond t...

Discoveries in the last few years have revolutionized our knowledge of the universe and our ideas of its ultimate fate. Measurements of the expansion of the universe show that it is not slowing down under normal gravity but accelerating due to an unknown, gravitationally repulsive “dark energy”. This may be a clue to new properties of quantum physics or of gravity beyond Einstein. I present an ...

Because cosmology touches our ideas about what lies “beyond” as much as about what went “before”, it will always raise controversy and even cause bitterness. Although I am a classical astrophysicist, I reckon that even in the solar spectrum, some elements† indicate some general principle of the universe. At the same time, another universe than that of the astrophysicists is offered by mathemati...