Cosmologists established their vaguely defined, astoundingly precise standard model, Lambda–cold dark matter, by analyzing the tiny variations in the temperature of the cosmic microwave background, mapped above by Europe's Planck spacecraft.Discovered less than a century ago, the expansion of the universe causes galaxies to rush away from Earth, stretching their light to longer, redder wavelengths.
The model, called Lambda–cold dark matter , is vague. It assumes dark energy is just a property of empty space called a cosmological constant, Lambda, and dark matter is just cold invisible stuff, hence CDM. But the model fits cosmological data beautifully, especially those from the CMB. The temperature of the CMB varies ever so slightly across the sky, providing a blotchy snapshot of the universe at the moment neutral atoms formed, 380,000 years after the big bang.
One solution assumes dark energy isn’t a cosmological constant, but is due to some kind of new physics. If so, its concentration could have fallen or even grown as the cosmos evolved, resulting in an expansion history that could start as the Lambda- CDM predicts but end at the higher directly measured value. In fact, Riess says he started his project in hopes of finding just such an effect.