It was only a little over a century ago that the science of cosmology was born. Out of the radical ideas of Albert Einstein and the observational evidence that space is expanding, the modern cosmological paradigm known as the Big Bang emerged. For the first time in history, human beings had begun to understand how their Universe began.


After decades of observation and measurement, we now know in some detail how our Universe expanded and evolved over all but the first moments of its history.

Any number of observations have confirmed the predictions of the Big Bang theory to an incredible, and frankly unexpected, extent. The rate at which our Universe has expanded over the past 13.8 billion years agrees with the equations derived from Einstein’s theory of general relativity almost one hundred years ago, and measurements of the large-scale distribution of galaxies and galaxy clusters is indistinguishable from that which the theory predicted.

And most impressive of all, the detailed pattern of temperature variations observed across the cosmic microwave background has been a treasure trove for cosmologists, revealing to us everything from the amount of matter present in our Universe, to the large-scale geometry of space itself.

From a few hundred thousand years after the Big Bang to the present, we have a rich array of observations and measurements to rely on, and this collection of data has left us confident that we understand this portion of our Universe’s history quite well.

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Along with the vast majority of other cosmologists, I would be shocked if this part of the chronology turned out to be substantively wrong – there are just too many different and powerful lines of evidence that support our current understanding of this series of events.

Finding out that we got this very wrong would be like finding out that there had never been an American Civil War, or that Christopher Columbus actually landed in Wales in the 12th Century and not in the West Indies in 1492. While it’s good to keep an open mind about what you might have gotten wrong, in some cases the evidence is just too strong to reasonably contemplate being entirely mistaken.

But as we go back further in cosmic history, our confidence begins to decline. Between the first few seconds and a few hundred thousand years after the Big Bang, we still have fairly substantive support for what’s described in the standard timeline.

Observations and measurements tell us that the rate of expansion and the quantities of matter and energy in our Universe cannot have been very different from those our calculations predicted. That said, it is still plausible that important and unknown cosmological events may have taken place during this period.

The information we have about our Universe’s first hundreds of thousands of years is significant, but it is not exhaustive.

The mysteries of the Universe’s first seconds

Reaching back even further – into the first seconds and fractions of a second after the Big Bang – we transition from having incomplete information to having essentially no direct observations to rely on. This era remains hidden from our view, buried beneath impenetrable layers of energy, distance and time.

Our understanding of this period of cosmic history is, in many respects, little more than an informed guess, based on inference and extrapolation.

Despite all of the successes of modern cosmology, there is much about our Universe that remains unknown. The most famous of these mysteries is that of dark matter.

Astronomers and cosmologists have determined how much matter there is in our Universe to very high degree of precision, and it is much more than exists in the form of atoms. After decades of measurement and debate, we are now confident that most of the matter in our Universe does not consist of any known substances, but of something else that does not appreciably radiate, reflect, or absorb light.

Over the past few decades, physicists have been engaged in an ambitious experimental program seeking to reveal what this substance is and how it was formed in the Big Bang. But despite initial optimism, we remain ignorant of dark matter and its nature.

The experiments have performed just as designed, but have seen nothing. Dark matter has turned out to be far more elusive than we had once imagined, forcing us to give up many of our favourite theories, and to consider radically new ideas for what the dark matter might be, and for how it may have formed in the first instants after the Big Bang.

Even the origin of “ordinary” matter harbours stubborn secrets of its own. Although protons, neutrons and electrons, and the atoms they constitute can be easily created through well understood processes, such processes also create an equal quantity of more exotic particles, known as antimatter.

Whenever particles of matter and antimatter are brought into contact with one another, both are annihilated. So why, then, does our Universe contains so much matter and so little antimatter? In fact, why is there any matter at all?

If matter and antimatter had been created in equal amounts in the heat of the Big Bang – as our current understanding of physics would lead us to expect – then almost all of it would have been destroyed long ago, leaving our Universe essentially devoid of atoms. Yet there are atoms all around us.

Somehow, more matter than antimatter must have been created in the first fraction of a second of our Universe’s history. We don’t know how or when this came to pass, or what mechanism was responsible. But somehow, something about the conditions of the early Universe made it possible for the seeds of atoms – and all of chemistry, including life – to survive the heat of the Big Bang.

Going back even further in time, we come to what is perhaps the single most intriguing of our cosmic mysteries. In order to make sense of our Universe as we observe it, cosmologists have been forced to conclude that space, during its earliest moments, must have undergone a brief period of hyperfast expansion.

Reader Q&A: What is dark energy? Asked by: Fred Thomas, London During the 1990s, astronomers measuring the rate at which the Universe is expanding made a shock discovery: it’s actually accelerating, as if the whole cosmos is being propelled by some invisible source of energy. This is so-called dark energy and its origin is one of the deepest mysteries in science. Various explanations have been put forward, with arguably the simplest being that it’s a manifestation of so-called quantum vacuum processes. According to the laws of the subatomic world, there is always some uncertainty about the amount of energy filling even empty space. This vacuum fluctuation energy has been detected in the lab, and theorists have shown it can have the ‘anti-gravitational’ effects of dark matter. So far, however, they have struggled to produce a detailed theory of its cosmic effects. This has led to suggestions that dark energy may simply be a force-field left over from the Big Bang. Sometimes called quintessence, it’s capable of getting stronger over time, but again details remain elusive. There have even been claims that dark energy is leaking out from hidden extra dimensions of space that failed to expand following the Big Bang. Until there’s a breakthrough in the underlying theory, however– all this is little more than speculation. Read more: What is dark matter?

Could ‘dark matter’ just be dead stars and planets floating in the depths of space?

How dense is the Universe?

Although this epoch of inflation lasted only a little longer than a millionth of a billionth of a billionth of a billionth of a second, it left our Universe utterly transformed. In many ways, one can think of the end of inflation as the true beginning of the Universe that we live in.

Despite identifying many compelling reasons to think that inflation really took place, cosmologists still know and understand very little about this early, key era of our cosmic history.

And lastly, in the 1990s, cosmologists set out on an ambitious program to measure the more recent expansion history of our Universe, allowing us to determine the geometry and ultimate fate of our world. For the first time, it was thought, we would be able to learn whether our Universe will continue to expand forever, or instead eventually reverse and collapse in upon itself.

These measurements were ultimately successful, but they revealed to us something that very few scientists expected: our Universe is not only expanding, but is expanding at an accelerating rate.

To explain this fact, we have been forced to conclude that our Universe contains vast amounts of what is known as dark energy, filling all of space and driving it apart. But our best efforts to understand this phenomenon have come up almost entirely empty handed. We simply do not understand what dark energy is, or why it exists in our Universe.

Each of these puzzles is deeply connected to the first moments that followed the Big Bang. Whatever the dark matter consists of, it was almost certainly formed during our Universe’s first fraction of a second.

Similarly, the simple fact that atoms exist in our world reveals that these earliest moments must have included events and interactions that we still know nothing about. Cosmic inflation also took place during these earliest of times, and could be connected to the existence of dark energy, raising many questions of its own. In these and other ways, our Universe’s greatest mysteries are firmly tied to its first moments.

How we’ll find out

Although our Universe’s first moments have proven challenging to study, this has not prevented us from trying. Scientists are currently in the process of building telescopes that will measure in new ways and with greater precision the light known as the cosmic microwave background.

These measurements, it is hoped, will enable cosmologists to learn more about inflation, such as when it took place and what forms of energy may have powered it. In the more distant future, space-based gravitational wave detectors will provide us with a new way of seeing the early Universe, potentially revealing signals from inflation, as well as from any phase transitions that may have taken place during our Universe’s the first moments.

One day, we may even begin to detect and study the particles known as neutrinos that were produced in the Big Bang. Difficult, yes. But impossible, no.

It is clear that our greatest cosmic mysteries are tied to our Universe’s first moments. How did our Universe come to contain so much matter and so little antimatter? How was the dark matter formed? Our Universe seems to have undergone a brief period of hyperfast expansion, but how and why? And is this connected to the fact that our Universe is now once again expanding at an accelerating rate?


Today, these are open questions. But today’s mystery is tomorrow’s discovery. Powered by new data, observations and ideas, we are poised to shed light on these perplexing questions. And with that light, we will see deeper and more clearly into the past than ever before – closer to the edge of time.