Astrophysics for People in a Hurry

Cosmic microwave background is made of photons that were created early in the universe when it had expanded enough for atoms to form and to release photons. These photons have traveled nearly 14 billion years across an expanding universe; that expansion has cooled them off to a mere 1,000th of the energy they had when they were formed. When they strike our detectors, their frequency is in the low-energy microwave band. Slight variations in CMB temperatures from different regions of space enable scientists to diagram the shapes in the mass of the early universe, shapes that comport with the superclusters of galaxies we detect.

CMB gave cosmologists their first real tool to decipher the universe’s history, “bringing cosmology from a nursery of clever but untested ideas into the realm of a precision, experimental science” (56). 

Einstein’s 1916 theory of general relativity describes gravity as a warpage in the space around objects. Large masses curve space more, somewhat in the way that a bowling ball suspended on a rubber sheet warps the sheet. All that gravity would cause the universe to collapse, so Einstein added to his theory a “cosmological constant” (99), also called lambda, that pushed back against gravity and kept the universe stable. Then astronomer Edwin Hubble in 1929 reported the universe is expanding. This made lambda unnecessary, and Einstein, chagrined, removed it.

In 1998, though, scientists discovered the universe’s expansion is happening faster and faster, and adding back Einstein’s lambda accounts perfectly for the accelerating expansion. This force, not yet understood, is called “dark energy.” The story of its discovery is a testament to the relentless, truth-seeking process of science, whose practitioners must sometimes abandon their cherished beliefs and choke down their pride to acknowledge where the search for truth sometimes leads.

Cosmology is the study of the cosmos, or universe, including its formation, history, and future. For centuries a part of metaphysics, the branch of philosophy that studies the nature of reality, cosmology today has acquired scientific tools that help test its theories and make predictions. The first major tool was the cosmic microwave background (CMB), the ancient photons whose light gives distinctive clues to the shape of the early universe. Much of the book, especially its recounting of the history of the universe, discusses cosmology from the perspective of an astrophysicist.

The Big Bang caused the universe to spread apart rapidly; today, after 14 billion years, that expansion is speeding up instead of slowing down because of gravity. The cause of this speed-up isn’t known; it’s called “dark energy,” and it amounts to nearly two-thirds of all the matter and energy in the universe. (See also “Cosmological constant,” above.) Dark energy represents one of the many humbling proofs that scientists don’t know everything and that the universe has marvels yet to reveal.

Scientists have discovered that the visible matter in the universe is only one-sixth of the amount required to keep stars and galaxies from flying apart. Observers also have found that most of the mass of the universe consists of material that exerts its gravity on visible matter but otherwise doesn’t interact with it. This “dark matter” also precisely makes up for the missing mass needed to hold galaxies together.

Though the dark-matter puzzle has frustrated science for decades, “[p]article physicists are confident that dark matter consists of a ghostly class of undiscovered particles that interact with matter via gravity, but otherwise interact with matter or light only weakly or not at all” (90). The trick is to detect the nearly undetectable; the true nature of dark matter, then, remains to be deciphered. As with dark energy, the mystery of dark matter mocks our pretensions to vast knowledge; it’s a reminder that we still have a long way to go before we fully understand our cosmos.

The electromagnetic spectrum is the entire range of electromagnetic radiation distributed according to the frequency and wavelength of the radiation energy, which is comprised of photons. Photons are massless particles that transmit energy and travel at the speed of light. Light is the visible type of photon, but there are many other types that vary depending on how energetic the photons are. Low-energy photons—radio waves, microwaves, infrared—have low frequencies, which is the number of times they vibrate per second; high-frequency photons—ultraviolet, X-rays, and gamma rays—are very high energy. In the middle are the photons that transmit visible light, including the colors we can see: red, yellow, orange, all the way up to violet. (See “Light” and “ROYGBIV,” below.)

The electromagnetic spectrum is the source of much, if not most, of our knowledge of the universe as a whole and the knowledge described in this book.

A galaxy is a very large collection of stars. The average galaxy in the universe contains more than 100 billion stars, and there are roughly 100 billion galaxies. This amount reminds us of the overwhelming size of our universe; it both humbles and inspires us and is a large part of why the author loves his work.

Light is the visible form of photons. It occurs in different colors, depending on how energetic the photons that transmit them are. Red is the least energetic and has the lowest frequency, or number of waves per second. After red in order of lowest to highest frequency are orange, yellow, green, blue, indigo, and violet. This sequence is abbreviated ROYGBIV, for the first letter of each color. Below visible light is infrared, as in heat lamps; above visible light is ultraviolet, or UV, the light that can cause a sunburn. For all its range of colors, visible light is but one small section of the entire electromagnetic spectrum that science uses to figure out the nature of reality and the size and contents of our universe.

Light travels through space at 186,000 miles per second. A light-year is the distance light travels through space in a year, roughly six trillion miles. The nearest star system to our Sun is Alpha Centauri, which is about four light-years away; the farthest galaxies are tens of billions of light-years distant. As far as we know, nothing can travel faster; “We will never outrun a beam of light” (41). Even these speediest particles take billions of years to cross the universe.