Georges Lemaître: The cosmic mystery that remains at the heart of our universe after the big bang

Google is celebrating Georges Lemaître's 124th birthday with a special doodle. But even after all this time, his greatest discovery is an enthralling mystery.

Lemaître, a Catholic priest who died in 1966, is best known for first proposing the Big Bang theory of the universe, which explains how everything around us came to be. He knew it as the "cosmic egg" – its more famous name was actually coined to mock the idea, and has been criticised for giving a misleading idea of what happened.

But just as thrilling as the cosmic egg is what it suggested: that the universe has been expanding ever since. It is expected to keep expanding forever, to the point that it becomes so expanded its central processes break down.

The mechanisms behind this process remain mysterious, with cosmologists suggesting that it is being propelled by unknown dark energy. But there is a greater mystery at the heart of the idea, too.

Though scientists know an incredible amount about that expansion, and have studied in fascinating detail the rate at which it is happening, it is still not clear how quickly that expansion is happening. And that is not because we do not know enough – rather because the vast amount we do know does not seem to agree.

In fact, as scientists get more precise measurements of that rate, known as the Hubble constant, they become more confused. And recent ultra-precise measurements have suggested that some mysterious, new and undiscovered physics might be at work in the universe.

Repeated findings from measurements using technology such as the Hubble Space Telescope have found strange discrepancies in the speed of expansion. Repeated findings have discovered that the universe appears to be expanding faster now than it was just after the big bang.

The strange discrepancy looks like a mistake. But it becomes more clear as the results become more precise that it is in fact "not a bug but a feature of the universe", as Nobel Laureate Adam Riess has put it.

The discrepancy – which relies on work done by Lemaître decades ago – is something like measuring the height of a child when it is three years old, then meeting the same child again when it is older and finding that it is far more tall than you would ever expected it to be.

A whole host of explanations for this strange effect have been proposed. Almost all of them rely on the 95 per cent of the universe that remains dark and mysterious to us – the parts of the cosmos that we know through theory that are there, but which we have never directly seen.

The strange effect could be the consequence of dark energy, which scientists already think is powering the expansion of the universe. It might be that dark energy is pushing galaxies apart with more strength, suggesting that the Hubble constant, so important to Lemaître's work, is not really a constant at all but changes through time.

Or it could be caused by a mysterious subatomic particle, travelling through the universe at the speed of light, but which we have not yet seen. Yet another suggestion is that dark matter could interact with normal matter in ways we didn't understand before.

Scientists don't know which of these is the case – or if the explanation is something else entirely. But it does appear that there is something very strange and as yet undiscovered happening in the universe, acting as it expands.

Whatever it is will sheds new light on the expansion of the universe – an already stunning discovery when it was first proposed by Lemaître, and one that becomes all the more awe-inspiring as we learn more about it.