The standard model of cosmology is our picture of the universe on a grand scale. Its basis is Einstein’s theory of gravity, the general theory of relativity, which he formulated almost exactly a century ago. To formulate the model, though, Einstein and others had to make some key assumptions about the way the universe ticks – laws that are the foundation stones of our cosmic understanding.
LAW 1: The speed of light is a constant
Nothing can exceed this cosmic speed limit
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Back in the 1860s, James Clerk Maxwell was melding electricity and magnetism into one unified theory of electromagnetism. But however he sliced the equations, they only made sense if light travelled through space at the same constant speed, regardless of the speed of its source.
This is odd. If someone fires a bullet from a moving car, to a bystander the bullet travels at the sum of its speed and the car’s speed. Yet when 20 years later US physicists Albert Michelson and Edward Morley were looking for the luminiferous ether, a medium supposed to carry light, they reached the same conclusion: however you look at it, the speed of light is a constant.
Not only that, it is the ultimate cosmic speed limit. No influence – not matter, not information, not gravity or any other force – may travel faster than it. Reports of cosmic speed breakers, such as faster-than-light neutrinos announced in 2011, have always turned out to be wrong. Einstein raised the constant speed of light to a principle of nature, and began to rebuild physics around it – the starting point of his twin theories of relativity, special relativity and general relativity.
Special relativity
Motion, space and time are all relative
As Einstein worked out, the principle of a constant speed of light has some odd consequences. In everyday experience, two cars approaching each other at 100 kilometres an hour collide at double that, 200 km/h. But imagine you’re sitting in one of two spaceships approaching each other, each travelling at 90 per cent the speed of light, c. From the perspective of one, what speed is the other approaching at?
The exact figure doesn’t matter* – but it can’t be bigger than c. In Einstein’s special theory of relativity of 1905, time and space warp to accommodate light’s speed limit. Moving clocks tick slower and moving rulers appear shorter, so there is no one objective measure of time and space – and you really will age less in a speeding spacecraft. At our normal speeds, these warping effects are negligible, but close to light speed they become hugely significant, and ensure no object can ever cover a given space in a shorter time than light can.
* It’s 99.4 per cent of the speed of light
E=mc2
This most famous equation of physics stems from special relativity, and says that mass is just a concentrated form of energy, connected by the constant speed of light. So bash particles together at very high energies, as in CERN’s Large Hadron Collider, and you can create other, more massive particles – a path of discovery that, by probing newly developed quantum field theories, eventually led to the standard model of particle physics.
Reality guide: How everything fits together
The six principles that rule the universe… and the six big problems we still can’t crack
LAW 2: The equivalence principle
Gravity and acceleration always look the same
In the 16th century, Galileo noticed that falling objects accelerated at the same rate regardless of their mass. A feather and a hammer dropped from the Leaning Tower of Pisa will hit the ground at the same time, once you discount air resistance. During the Apollo 15 lunar landing, astronaut David Scott confirmed that principle on the airless moon.
Newton showed that this could only be true if an odd coincidence held: inertial mass, which quantifies a body’s resistance to acceleration, must always equal gravitational mass, which quantifies a body’s response to gravity. There is no obvious reason why this should be so, yet no experiment has ever prised these two quantities apart. As with light’s constant speed, it was Einstein who declared this equivalence a principle of nature.
LAW 3: The cosmological principle
The universe is the same in all places and in all directions
A few decades before Galileo, Copernicus dared to suggest that Earth was not a special place in the cosmos. A century or so later Newton in his great treatise Principia assumed that the solar system was embedded in a uniform space that extended vast distances in all directions.
These are the origins of what in modern cosmology has morphed into the cosmological principle: gaze out into the universe and everything is more or less the same everywhere and in whatever direction you look. Local clumps of matter exist in the form of solar systems, galaxies and clusters of galaxies, but on a big enough scale everything averages out to uniformity.
It’s a simplification that makes the mathematics a lot easier when we’re trying to build a working model of the cosmos. But our limited view makes it difficult to say whether it truly is a universally valid principle. The discovery of ever-bigger structures, for example the whopping 10 billion-light-years-wide arc of galaxies seen in 2013 and dubbed the Hercules-Corona Borealis Great Wall, is calling that into question.
General relativity
Einstein’s warped theory of gravity
Special relativity tells us motion warps space and time. So does acceleration – and gravity is a form of acceleration. That’s the lesson of Einstein’s magisterial general theory of relativity of 1916, which combines special relativity and the equivalence principle into our working theory of gravity. Massive objects bend space and time (collectively known as space-time) around them, making things appear to accelerate towards them.
General relativity provides a framework to explain the large-scale workings of the universe, but a cosmological model requires a further crucial piece of information: how matter is distributed.
Gravity
Gravity explains why we feel a downwards pull towards Earth and why Earth orbits the sun. Although dominant over large cosmic scales and near very large masses like planets or stars, it’s actually by far the weakest of the four known forces of nature – and the only one not explained by quantum theory.
Gravitational waves
These ripples in space-time were the last unconfirmed prediction of general relativity until they were finally spotted in September 2015. The signal of two massive black holes spiralling together and merging was a triumph of painstaking, patient detective work by the Advanced LIGO experiment.
THE STANDARD MODEL OF COSMOLOGY
When Einstein first used general relativity to build a cosmic model, he followed the orthodoxy of the day and assumed the universe was static: neither expanding nor contracting. Observations in the 1920s, however, showed that distant galaxies are “redshifted” as if they are moving away from us. Others then used his theory, plus the simplifying cosmological principle that the universe’s matter is uniformly distributed, to build models of an expanding universe. This was the beginning of today’s standard cosmological model, which describes a universe that began in the hot, dense, infinitesimal pinprick of the big bang some 13.8 billion years ago.
The cosmic microwave background
Discovered by accident in 1964 as a background hiss in a gigantic telecoms receiver, this cool sea of radiation is now seen as clinching evidence of the big bang described by cosmology’s standard model. The oldest light in the universe, it was sent on its way some 380,000 years after the big bang, when the cosmos had cooled sufficiently for the first atoms to form, allowing photons to travel freely. Probes collecting this light, most recently ESA’s Planck mission, have mapped it in fine detail, providing information on the universe’s earliest years and its make-up today.
Despite all its successes, the standard model of cosmology conjures up phenomena such as dark matter and dark energy. Read more about these mysteries and more in Reality guide: Six problems physics can’t explain
Or to explore three more fundamental principles that inform our picture of reality on the smallest scales, see Reality guide: The essential laws of quantum physics
