What existed before the Big Bang?

The universe is, in fact, thought to have emerged from a sphere of gas smaller than a pea.

To call the beginning of the universe the "Big Bang" (science shorthand for that most distant moment to which one can still trace the operation of our laws of physics) is to use something of a misnomer. Current evidence suggests that, far from being "big", the whole, vast expanse of space and all the visible galaxies and stars originated in a dense sphere of glowing gas much smaller than a pea. Some cosmologists, affecting a familiarity with events so far removed from our everyday experience, refer to the beginning now as just the "Bang", while general relativists – the scientists who study the physical consequences of Einstein's 1915 theory of relativity – draw a line at the base of their blackboards and say: this is the singularity where it all started.

The era of the universe we live in began about 14 billion years ago, when all that we can see today was compressed to a very high density and pressure, a plasma hotter than that in the core of a star. The observed elemental composition of the universe – especially the amount of helium present – is our best evidence for the first few seconds of its existence.

Another sign of the first fractions of a second of time after a "beginning" is the smoothness and uniformity of microwaves that fill the cosmos. The rules of Einstein's theory of general relativity deeply interconnect space and time to mass and energy. Matter can, in a sense, create the space it expands into and generate the time in which to do it.

The universe was born in a state of very low entropy, which gives time its forward arrow and its enormous impetus to evolve. To visualise this, picture an alternative universe born into a state of high entropy – imagine the universe as a lukewarm, uniform gas, evenly spread throughout a large box. Viewed day after day, the gas molecules in the box would be seen to bounce around, but the overall picture of a uniform gas in a box would remain unchanged, with no evolution of the overall temperature or distribution of the gas anywhere in the box. A low-entropy state, on the other hand, is like a box which is empty (a vacuum) except for a concentrated ball of hot gas in one corner. This situation is not stable, and the ball of hot gas will expand quickly to fill the entire volume of the box, cooling as it goes.

There is a similar analogy to the Big Bang early universe, except that there is no "empty box" when the universe starts to expand; instead, the mass-energy of the universe creates the space-time to expand into, as it evolves.

Was there an era before our own, out of which our current universe was born? Do the laws of physics, the dimensions of space-time, the strengths and types and asymmetries of nature's forces and particles, and the potential for life have to be as we observe them? Or is there a branching multi-verse of earlier and later epochs filled with unimaginably exotic realms?

We do not know.

Illustrated by Josh Cochran

joshcochran.net

Written by Brian Yanny PhD

Research scientist, Fermi National Accelerator Laboratory, Illinois

What is the origin of the moon?

The capture theory on the moon's origins is back in vogue.

Until the Apollo landings in 1969, there were three theories about the origin of the moon: the capture theory, the fission theory, and the double-planet theory. The capture theory proposed that the moon formed elsewhere in the solar system and was captured by Earth's orbit as it travelled by. The fission theory proposed that the moon was spun out of the earth during a period of rapid rotation early in Earth's history. The double-planet theory proposed that the Earth and the moon formed simultaneously from small proto-planets, or planetesimals.

When the Apollo astronauts brought back lunar samples, the basalts were found to be almost identical in composition to Earth's, while the moon's oxygen isotope ratios were identical to Earth's. The major differences were detected in some rare Earth-element abundances, the moon's near-total lack of water and volatile compounds, and its lack of a liquid iron core.

By 1984, these data from the Apollo missions formed a fourth theory. The collision theory, which is used in science textbooks today to describe the moon's origin, posits that during the tumultuous early days of the solar system a large proto-planet, approximately the size of Mars, collided with Earth, had already stratified into a core, mantle and crust. The resulting impact re-melting its crust and sending a plume of mantle material into space. The heavier ejected material remained in Earth's gravitational field and later coalesced to form the moon. This theory explains the relative lack of volatiles and water in lunar rocks, the similarities in chemistry of lunar rocks to Earth's mantle, and the lack of a substantial metallic core (since the Earth's core was not breached during the impact).

In the past decade, a number of mass spectrometry techniques have been developed to determine additional chemical and isotopic compositions of minerals, which have caused geologists to rethink the collision theory – either in its entirety, or to refine the parameters of the impact. For example, the 2010 and 2011 discoveries of measurable water in both lunar basalt glasses and in olivine inclusions is inconsistent with collision theory as it now stands.

The ages of zircons in some of Earth's oldest rocks also do not support the collision theory. Today,the capture theory is making a comeback with a subset of geoscientists, who see less and less geochemical evidence for collision in the rock record. The capture theory – although it requires a "right place at the right time" set of interplanetary conditions that physicists and astronomers find unlikely – does explain some of the geochemical and geophysical data that can't be explained fully by the collision model.

As the Apollo rocks are reanalysed using instrumentation that was not available in the 1970s, our ideas about the origin of the moon are guaranteed to evolve.

Illustrated by Lauren Nassef

laurennassef.com

Written by Sarah K Carmichael PhD

Assistant professor, Appalachian State University, North Carolina

What is dark energy?

About 73% of the universe is made of dark energy.

In 1998, astrophysicists were shocked when new data from supernovae revealed that the universe is not only expanding, but expanding at an accelerating rate.

Until then, it was widely believed that the rate of expansion was slowing, due to the gravitational attraction of ordinary and dark matter in the universe. To explain the observed acceleration, a component with strong negative pressure was added to the cosmological equation of state and called "dark energy".

A recent survey of more than 200,000 galaxies appears to confirm the existence of this mysterious energy. Although it is estimated that about 73% of the universe is made up of dark energy, the exact physics behind it remains unknown. The simplest explanation, known as the "cosmological constant", is that dark energy is the intrinsic, fundamental energy of a volume of space, filling it homogeneously. Other models, such as "quintessence", propose that dark energy is more dynamic and can vary in time and space. Common to both models, however, are the assumptions that dark energy is not very dense and interacts only with gravity – two properties that make it extremely difficult to detect in the laboratory.

Illustrated by Ben Finer

benfiner.com

Written by Michael Leyton PhD

Research fellow, Cern

What is antimatter?

Antimatter is matter composed entirely of antiparticles.

For every known particle there exists an associated antiparticle with the same mass and opposite electrical charge. The antiparticle of the electron is the positron, for example, and the proton and antiproton make a similar pair. Matter composed entirely of antiparticles is called "antimatter". In the same way that a proton and an electron form a normal-matter hydrogen atom, an antiproton and a positron form an antihydrogen atom.

Antihydrogen atoms are more than hypothetical; nine were created for the first time in 1995 by physicists at Cern (the Geneva-based laboratory for particle physics). However, these atoms are difficult to study experimentally, because they are quickly annihilated when they come into contact with matter.

At present, there is no experimental evidence of any significant concentration of antimatter in our observable universe. In other words, the universe we live in consists almost entirely of matter. This phenomenon is puzzling to physicists, given the symmetry between matter and antimatter.

Many competing theories attempt to explain how such an asymmetry could have come about. One group of theories focuses on understanding how nature, at the particle level, might favour certain matter reactions in comparison to their antimatter counterparts. Such reactions have been observed and studied extensively in the laboratory, but we do not know whether they alone can explain the matter imbalance in the universe. Other theories propose that there are indeed regions of the universe composed primarily of antimatter (a so-called antiuniverse), but that these regions are widely separated from matter-dominated regions or are possibly outside of our visible universe. After all, there may be more to the universe than can be seen from Earth!

Illustrated by Leif Low-Beer

leiflow-beer.com

Written by Michael Leyton PhD

Research fellow, Cern

How are stars born? And how do they die?

Stars are born in nurseries.

Stars are born in galactic nurseries of cold, dense, dark gas clouds of molecular hydrogen, carbon monoxide and other simple compounds. Gravity pulls blobs of gas inward until they break off into roughly spherical lumps the size and mass of several solar systems. The centre of each lump condenses and heats to the point where the basic nuclear fusion reaction of hydrogen into helium is initiated, creating a central star, while further out, planet-sized chunks of gas and dust coalesce and orbit the star.

Once the hydrogen of a star's core has been fused nearly entirely into helium (a process that takes 10 billion years for a star with the mass of our sun), the star begins to fuse heavier and heavier elements rapidly until iron is reached, at which point no further nuclear energy can be efficiently extracted. Gravity, which has been held at bay until this point by the central nuclear furnace, then takes over, pulling elements toward the centre and crushing the star's core. If the core is less massive than several times the mass of our sun, the star becomes a white dwarf, held together by degeneracy pressure, the same force that keeps electrons separated in their orbits around atomic nuclei. Such a white dwarf will cool off over trillions of years, eventually becoming a black dwarf, a dark lump floating through space, nearly undetectable.

On the other hand, if the iron core is very massive, its one-second collapse results in an enormous supernova explosion and a push of core electrons and protons into each other, generating a spinning, pulsing neutron star, or, in extreme cases, a new black hole. The neutron star will slow and dim over billions of years, and the black hole will eventually evaporate, again on a timescale many times that of the age of our present universe.

llustrated by Jenny Volvovski

also-online.com

Written by Brian Yanny PhD

Research scientist, Fermi National Accelerator Laboratory, Illinois

Extracted from The Where, The Why and The How, published by Chronicle Books, £15.99. Buy it at a discount from guardianbookshop.co.uk