OBSERVABLE UNIVERSE
OBSERVABLE UNIVERSE
GLOSSARY
critical density If the universe has this energy (or matter) density then it will be perfectly balanced between forever expanding, and contracting in on itself in the far future in a ‘Big Crunch’. Current measurements of the universe suggest this is exactly the density it has.
dark energy A mysterious energy source in the universe, with negative pressure, that can speed up cosmic expansion. No one knows what it is but all constraints suggest it is constant with time and space (the ‘cosmological constant’).
Eddington limit Light causes a (small) pressure. The Eddington limit is the maximum luminosity a star can have before it would blow itself apart by its own radiation pressure.
Einstein’s field equations The set of equations (which can be written as a single ‘tensor’ equation with ten independent parts) that describe how matter creates gravity in Einstein’s general theory of relativity.
FLRW metric (also known as the Friedmann–Lemaître–Robertson–Walker metric) A metric is a solution of Einstein’s field equations. This particular solution applies to a homogeneous, isotropic and expanding universe.
general theory of relativity The theory of gravity published by Albert Einstein in 1915. While it is often described as ‘proving Newton wrong’, it actually reduces to Newtonian gravity in low gravity regimes, but this improvement can also explain gravity at the highest densities, and across the largest scales.
graviton A postulated particle that transmits information about the force of gravity.
hyperbolic paraboloid This is the name of a shape and is most familiar as the shape of the saddle of a horse. It is a surface that curves up in one direction and down in another. The hyperbolic paraboloid is the shape space would have in an open universe.
particles and antiparticles Particles and antiparticles have the same mass, but opposite charge or magnetic fields, and if they get close to one another they will annihilate into pure energy.
photon A particle of light. The energy of a photon is proportional to its frequency, so high frequency (short wavelength) photons like X-rays carry a lot of energy, while low frequency photons carry very little.
positive cosmological constant A constant that can be added to Einstein’s field equations to counteract the natural tendency for the universe to contract or expand otherwise (depending on its matter/energy density). This is often used as the explanation for ‘dark energy’.
quantum gravity A field of physics that is working towards unifying quantum mechanics and general relativity. Quantum gravity is necessary to explain the Big Bang, and the singularity in the centres of black holes.
quantum mechanics A branch of physics that is particularly important for things happening on the smallest, subatomic scales. In this theory, all particles can be described as waves, and you cannot ever precisely know both a particle’s location and motion.
GENERAL RELATIVITY
the 30-second blast
Albert Einstein published his general theory of relativity in 1915; it was, perhaps, his greatest scientific achievement. Space, he realized, is not merely the void where objects are absent; rather, it is the matrix within which everything in the universe resides. Even where there is no matter, space has three-dimensional form; and where there is matter, space will curve towards it, creating a dimple where the shortest distance between two points actually follows a curved path in space. Even more remarkably, Einstein understood that time is a dimension, similar to length, width and height in space. What we perceive as the passage of time is essentially our travelling in the dimension of time – albeit always forward, never back. Time comes together with the three dimensions of space to comprise four-dimensional space-time – the ‘fabric’ of the cosmos. The path to this discovery was tortuous – Einstein spent years just learning the required mathematics to express the theory correctly. The result, though, was a fundamental change to humanity’s understanding of the universe. Today, general relativity plays a role in both the mundane and the grandiose, from the accuracy of GPS locators in mobile phones to the trajectories of satellites and distant space probes to the extreme environments around supermassive black holes.
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Space and time are enmeshed as a four-dimensional fabric that can be bent and curved by objects with mass.
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The first observational confirmation of general relativity came in 1919, four years after Einstein first published on the topic. Sir Arthur Stanley Eddington, the most distinguished astrophysicist of his time, organized expeditions to observe the positions of stars in the direction of the Sun during a total solar eclipse. The stars appeared to be at slightly different locations than they would at night, thus confirming the curvature of space-time by the Sun’s mass.
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3-SECOND BIOGRAPHIES
ALBERT EINSTEIN
1879–1955
German-born theoretical physicist who contributed some of the most significant theoretical discoveries of modern physics, and his life and writings touched many broad areas of science and society
SIR ARTHUR STANLEY EDDINGTON
1882–1944
British astrophysicist; the Eddington limit, which concerns the maximum luminosity of cosmic energy sources, is named after him
Stars, planets, spacecraft and even people create dimples in space-time: that’s gravity.
COSMIC CURVATURE
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At school we learn that the shortest distance between two points is a straight line. If we draw a dot at one corner of a rectangle and another dot at the opposite corner, we can measure how far apart the dots are by drawing a diagonal line between them, the length of which is given by the well-known Pythagorean theorem: a2 + b2 = c2. In the universe, the shortest distance between two points is still a straight line. As Albert Einstein showed with the general theory of relativity, however, matter causes space to curve. Imagine drawing a box not on a flat sheet of paper, but rather on a distorting mirror; what does a ‘straight’ line look like now? Happily, measuring lengths across cosmic distances still follows mathematical rules. Einstein’s field equations set up the geometric framework, and they’re just slightly more complicated than the Pythagorean theorem. Depending upon the density of matter in space, and assuming no other external effects, a straight line across the universe could curve inwards upon itself (something like a ball) or outwards away from itself (like a hyperbolic paraboloid) or not curve at all if the density is exactly right – a value known as ‘critical density’.
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Measuring lengths in the universe is straightforward, albeit not exactly ‘straight’, thanks to Einstein’s field equations of general relativity.
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The Friedmann–Lemaître–Robertson–Walker (FLRW) metric, named after four scientists who derived it, is an exact mathematical solution to Einstein’s field equations that describe how matter, energy and space interact in the universe. The field equations themselves, in turn, are often written as a single mathematical formula that combines ten multicomponent equations into one. This one equation can lead to tremendously convoluted calculations, yet it has yielded surprisingly clear solutions that neatly explain the curvature of space-time.
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3-SECOND BIOGRAPHIES
ALEXANDER FRIEDMANN
1888–1925
GEORGES LEMAÎTRE
1894–1966
HOWARD ROBERTSON
1903–61
ARTHUR WALKER
1909–2001
Four scientists, Russian, Belgian, American and British respectively, who all helped derive the mathematical expressions that describe our curved universe; Friedmann and Lemaître worked separately, while Robertson and Walker worked together
‘Matter tells space-time how to curve, and space-time tells matter how to move.’
John A. Wheeler
COSMOLOGICAL CONSTANT
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As Einstein formulated his field equations that describe the way matter curves space-time, he noticed that the mathematical formulae implied the universe would either expand or contract. As it was important theoretically to allow for the possibility that the universe could remain the same size indefinitely, he included in the equations a counterbalancing term that could keep the cosmos static. That term became known as the cosmological constant. Several years later observations of distant galaxies made by Edwin Hubble and other astronomers showed that the universe was expanding and not static – a discovery that suggested the cosmological constant did not have to exist – so future generations of cosmologists simply set that term in Einstein’s field equations to zero. Einstein wrote that the constant no longer seemed necessary, and many have quoted him to have declared that it was his biggest blunder. A blunder, however, it was not. Further groundbreaking astronomical observations starting in the 1990s showed conclusively that the geometry of the universe is inconsistent with the expansion rate and mass density of the cosmos – and the best way to reconcile the anomalous behaviour is to reintroduce a non-zero cosmological constant. But what causes it? We don’t know.
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Something – expressed mathematically, perhaps erroneously, in Einstein’s field equations of general relativity as a constant value – is causing the universe to expand faster and faster.
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The current best measurement of the value of the cosmological constant is a small number but definitely not zero. It is within just the right range of values to counteract the combined gravity of all the matter in the universe, gently and gradually pushing outwards against the otherwise inward curvature. As the universe expands, the effect of the cosmological constant expands, too, so, in the absence of other forces, the cosmic expansion rate will grow forever.
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WENDY FREEDMAN
1957–
ROBERT KENNICUTT
1951–
JEREMY MOULD
1949–
Astronomers who led the Hubble Space Telescope Key Project that measured the cosmic expansion rate accurately enough to show that the universe has a non-zero cosmological constant
The Greek letter Lambda, inserted into Einstein’s field equation, represents the magnitude of the cosmological constant.
GRAVITY
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In 1687 Isaac Newton published his law of universal gravitation to attempt to explain how celestial objects move in the heavens. Gravity, he posited, is a pulling force that attracts any two objects separated by some distance. The strength of the force is proportional to the mass of each object but inversely proportional to the square of their distance. Newton’s theory of gravity has been an amazing scientific success: from grains of sand to clusters of galaxies, Newton’s formula is essentially correct. It wasn’t until a couple of centuries later, with the development of the general theory of relativity, that Albert Einstein improved on Newton’s model. Gravity, explained Einstein, is the curvature of space and time by objects that have mass. Quantum mechanics adds another layer of complication to our understanding of gravity, and current ideas about quantum theory are incompatible with general relativity in important ways. Some cosmologists are working to develop a unified theory of quantum gravity, but so far without success. And the existence of gravitons, the hypothesized subatomic particles that carry gravitational force from place to place, has, as yet, not been confirmed.
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Isaac Newton’s formulation of gravity as a force of attraction is an excellent approximation for the actual phenomenon of gravity – the curvature of space-time by mass.
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On the size, mass and density scales on which we humans interact, the effect of gravity on the movement of objects is equivalent to an acceleration that would be caused by a force; relativistic corrections on Newtonian gravity are needed only for the finest of calculations – the orbits of global-positioning satellites, for example. However, Einstein’s equations must be used when calculating gravity in extreme cosmic environments, such as regions near neutron stars or black holes.
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3-SECOND BIOGRAPHY
CHARLES MISNER
1932–
American physicist who co-wrote the classic graduate textbook on gravitation (Gravitation, with Kip S. Thorne and John Archibald Wheeler) and laid down pioneering theoretical groundwork on general relativity, cosmology and quantum gravity
On human scales, it is impossible to distinguish between gravity as a force or as curvature in space-time.
MATTER BRIGHT & DARK
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Astronomers have discovered two different kinds of matter in the universe, normal, or ‘baryonic’, matter – which is what makes up all of us, the Earth, the planets, the Sun and every star in the universe – and a mysterious dark matter, about which we know very little except that it clumps. Only baryonic matter creates light, which it does in a variety of different and well-understood ways, but both types create gravity, and we can measure their existence by the impact this gravity has on other objects nearby. We know that galaxies have far too much gravity to be comprised only of visible matter and that galaxies in clusters are moving so fast the cluster would fly apart if they were made only of what we can see. For decades physicists and astronomers have searched for a hint of the nature of this invisible matter, but as yet no particles of it have been directly detected. While we do know that a small fraction might be normal matter that is just too cold to make light, the rest has to be something truly exotic, something outside of the normal range of particles found on Earth and beyond the Standard Model. Were it not, the universe would look very different.
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All matter creates gravity, and we can measure its mass, but only some makes light (what we call ‘normal matter’), and most of the matter in the universe is transparent ‘dark’ matter.
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Very few astronomers doubt the existence of dark matter, but what kind of particle it could be is becoming an increasingly curious puzzle. Many possible options from particle physics have been ruled out by careful observations. But it has to be something, because the Standard Model would just look too different if there was not a substantial amount of dark matter in the universe – in fact, non-baryonic matter outweighs baryonic matter by a factor of five.
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FRITZ ZWICKY
1898–1974
Swiss astronomer and so-called ‘father’ of dark matter who deduced its existence in the 1930s as part of his work on galaxy clusters at Caltech, the institute where he spent much of his professional career
Dark matter is revealed by its impact on matter that makes light – from the clustering of galaxies, to the seeds of all structure and bending of light.
ENERGY BRIGHT & DARK
the 30-second blast
Almost everything we know about the universe comes from collecting, recording and analysing electromagnetic radiation in its various forms – gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. Electromagnetic energy – or, simply, ‘light’ – is carried by subatomic particles called photons and can be produced by a huge variety of physical phenomena from the mundane (lighting a match) to the exotic (nuclear fusion in the Sun). Oddly, the largest reservoir of energy in the cosmos may be an invisible, albeit all-pervasive one. Starting in the 1990s astronomers began to find evidence that the universe has a positive cosmological constant. By 2010 that fact had been observationally confirmed, implying that the universe is expanding faster and faster. One possible explanation for this acceleration is the existence of a tiny bit of energy, not electromagnetic in nature and invisible to telescopes, linked to every bit of space. The density of this ‘dark energy’ would be so low that it would be imperceptible on even galactic scales; spread across the universe, however, the energy would be enough to counter the force of gravity across cosmic distances of billions of light years. Astronomers have been trying to detect dark energy directly with specialized observations for years now – but so far without success.
3-SECOND FLASH
While electromagnetic energy illuminates the universe for us to see, more than two-thirds of the overall content of the universe appears to be energy we cannot see at all.
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Expansion of the universe overall is controlled by the total and relative densities of matter and energy in the cosmos. If current cosmological models and measurements are correct, the universe will continue to expand ever faster until, billions of years from now, at any point in any galaxy only stars in that same galaxy and its immediate neighbours will be observable, as any galaxies further away will be travelling so fast that they will no longer be visible.
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SAUL PERLMUTTER
1959–
BRIAN SCHMIDT
1967–
ADAM RIESS
1969–
American scientists awarded the Nobel Prize in Physics in 2011 for their work on measuring the accelerating expansion of the universe – the primary evidence for the existence of dark energy
As the universe grows, the total amount of dark energy grows too, as does the speed of cosmic expansion.
