Cosmology has enjoyed a phenomenal revolution that commenced with the discovery of cosmic microwave background radiation in 1964 and that continues today as astronomers try to understand why much of the content of the Universe is mysterious and invisible. Einstein's Telescope is a highly accessible account of the modern approach to an ancient question: "What is the nature of the Universe?" Philosophers in antiquity debated that question, it took on a theological flavour in the Middle Ages, and it became available to direct observation only with the advent of space telescopes.
I place the start of modern cosmology at the 1919 total eclipse of the Sun. Sir Arthur Eddington's photographs, taken during this eclipse, of the positions of stars close to the eclipsed Sun showed that the amount of bending of starlight by the solar gravitational field agreed closely with the predictions of Einstein's general theory of relativity.
When Eddington presented his observations to the Royal Society in November 1919, the society's president, Sir Joseph Thomson, declared: "It is the greatest discovery in gravitation since Newton." On hearing that news, a reporter for The New York Times scurried along to Fleet Street to send a special cable to his editor. The paper ran the story with the headline "Eclipse showed gravity variation". Media coverage propelled Einstein to worldwide fame overnight.
Evalyn Gates gives a cutting-edge account of how gravity rules the Universe and what astronomers have discovered by seeing gravity in action on a gigantic cosmic scale. The book's title refers to gravitational lensing: what the Sun can do in terms of light bending is achieved on a spectacular scale by supermassive black holes, which are ubiquitous in the cores of giant galaxies.
Following in the footsteps of Einstein, who famously used thought experiments, I invite you to imagine the 1919 eclipse observations hugely scaled up: for this visualisation, in our mind we replace the Sun with a supermassive black hole, we substitute the remotest galaxies in the Universe for local stars, and we observe at night with a modern telescope. What might you expect to see?
The extraordinary answer to that question is this: the actual warp and weft of the fabric of space-time, which is caused by the supermassive black holes. Einstein's telescope, in which a stupendous black hole is the lens, brings extremely distant objects within the grasp of optical telescopes. To be sure, the images that have spent up to 12 billion years winging towards us are greatly distorted. But that's easily fixed: the spectrum of the light, which has a rich harvest of information about physical conditions and distance, is unaffected by the distortions of a fairground mirror. According to Gates, a gigantic cluster of galaxies can produce beautifully bizarre, almost kaleidoscopic, images from a lens sculpted out of space-time.
Gravitational lensing is a cosmological tool, sensitive to all types of mass, and that's important because most of the mass of the Universe is invisible. In our galaxy, astronomers have discovered stars that are black holes not by seeing them directly, which by definition is impossible, but by observing their gravitational pull on another star. For example, in a binary star system, in which a giant star pirouettes with a black hole companion, the duo blazes forth as an X-ray source. On the largest scales, the presence of dark matter of any kind is deduced from the dynamical postures of galaxies, from which astronomers weigh the Universe.
Clusters of galaxies are the largest and most massive entities in the Universe. In most clusters, the speed of individual galaxies is sufficiently high that, at first glance, one would expect the clusters to disperse: the gravitational attraction of the luminous mass is not sufficient to stop the cluster flying asunder. But clusters of galaxies are remarkably stable, with exotic dark matter providing ballast. The measured velocities of galaxies within the cluster become a tool to probe the gravitational field and the total mass responsible for gluing the cluster together.
Astronomers have known since the 1930s that most of the matter in galaxies and clusters is invisible. In the case of the largest clusters, as little as 2 per cent of the matter is visible. In modern times, cosmologists have made precise measurements of the amount of cold dark matter in the Universe: it accounts for about 23 per cent of the mass. Importantly, this matter is of a kind not yet known to physics.
With the discovery of the microwave background, astronomers quickly realised that the fossil radiation carried information on the content of the Universe and the origin of its structure. It took more than four decades to design and launch into space instrumentation that is capable of evaluating the cosmological parameters with a sufficiently high degree of accuracy. The Wilkinson Microwave Anisotropy Probe measured the age of the Universe as 13.7 billion years, showed that the geometry of the Universe is flat, and determined that 72 per cent of the Universe is in the form of dark energy. Less than 5 per cent of the mass and energy in the Universe is composed of atoms. These are stunning discoveries, results that set a new reference frame for all cosmological investigations by dramatically reducing the freedom for theorists trying to model the Universe. According to Gates: "Dark energy is the most compelling mystery in physics today."
Observations from the Earth's surface have also contributed to the rebirth of cosmology. Gates rates the Sloan Digital Sky Survey as the most influential survey in the history of astronomy. The numbers are impressive: the survey has been measuring thousands of galactic red shifts every night. Its images cover one quarter of the sky, and it has observed more than a million galaxies, to create maps of the structure of the Universe. The data reveal the conditions in the early Universe during the first billion years after the Big Bang.
The Sloan survey was one of two projects that tracked a type of supernova explosion that is a standard candle used to calibrate the extragalactic distance scale. The analysis of their brightness and distance produced the biggest surprise of all: the expansion of the Universe is accelerating. About 5 billion years ago the Universe changed gear, going through a cosmic jerk that has speeded up its rate of expansion. As Gates puts it picturesquely, billions of years passed in which the expansion was being slowed by the mutual gravitational attraction of the mass in the Universe, but then dark energy hit the gas pedal and the acceleration of the Universe took off. Here the story reconnects with Einstein, whose cosmology had an anti-gravity fudge, the cosmological constant, which has parallels with dark energy. And so we meet in passing Aristotle's fifth element, quintessence.
Einstein's Telescope brilliantly summarises half a century of progress in which physical cosmology moved from being just a handful of crudely determined numbers to a precision science. Gates' final message is that "all signs are pointing to something very different from anything we have ever imagined".
Evalyn Gates is senior research associate and assistant director at the Kavli Institute for Cosmological Physics at the University of Chicago, and lives in the leafy Hyde Park area of the Windy City. A theoretical physician and cosmologist and a campaigner for women's rights in science, she counts Barack and Michelle Obama, who live nearby, as personal friends.
Born in Philadelphia, she moved to Batavia in New York State when she was a child, and excelled at mathematics in school. She discovered a passion for physics as an undergraduate at the College of William and Mary in Williamsburg, Virginia. In 1990, she gained a PhD in particle physics before becoming a fellow at Yale. She later became chairman of the astronomy department and then vice-president for science and education at the Adler Planetarium in Chicago.
In her spare time, Gates dedicates herself to her husband and three children and counts hiking in the Colorado Mountains as her favourite hobby.
Einstein's Telescope: The Hunt for Dark Matter and Dark Energy in the Universe
By Evalyn Gates
Published 31 March 2009