CDM. This shortcut cold lambda dark matter It characterizes the cosmological model in which all our knowledge of the evolution of the universe is concentrated. Based on six criteria, you can describe that the universe has continued to expand continuously since the Big Bang 13.77 billion years ago. This means that the universe is constantly creating space, increasing the distance between any two points. In fact, a mysterious form of energy, dark energy – the capital letter lambda for short – has accelerated this process since the universe was 9 billion years old.
To see how fast the universe is expanding at any given moment, you can refer to a surprisingly simple empirical relationship: the farther away a galaxy is from us, the faster it is moving away from us. This relationship is linear: if a group of stars is twice as far from us as the first, they will fly away from us at twice the speed. George Lemaître and the American astronomer Edwin Hubble made this discovery about 90 years ago, and it is found in the Hubble-Lemaître law.
Then this law expresses the number by which you must multiply the distance to a particular galaxy to get the speed with which it is moving away from us. It is known as the Hubble constant. The name “constant” is a bit unfortunate, because as the universe expands faster and faster, its value is also constantly changing. But if you look at the universe at a particular point in time, let’s say now, the corresponding Hubble constant, denoted by H, but it should be the same in all places in the universe.
About H0 We have roughly two ways. Either you get the value of H From the cosmic microwave background, the afterglow of the Big Bang. Or you can take advantage of galaxies that know exactly how far they are and at what speed they are flying away. The Hubble constant is the product of velocity and distance.
You get the velocity from the redshift, which is the extent to which the wavelength of the star’s radiation extends as it moves away from us. And what about the distance? You can define it in the universe with two components. On the one hand there is the apparent brightness of the cosmic source. This is the easy part of the story. It is simply the brightness you measure when radiation hits your detector at the end of its journey.
On the other hand, absolute brightness, which is what really radiates the radiation source. You basically don’t know those, except for certain types of stars. You have Cepheids, stars with a periodically changing intensity of radiation. There is a clear correlation between the period of change and absolute brightness.
Now the difference between absolute and apparent brightness is a measure of distance: the longer the radiation travels between the source and the detector, the weaker the light that eventually falls. Since you can specify both the apparent and absolute brightness of Cepheids, they are perfect cosmic landmarks. More specifically, to determine the distance to nearby galaxies, after which the supernovae can take on the role of distance finder.
The first method, using the cosmic microwave background, tells you that a galaxy located 1 megaparsec away – 3.26 million light-years away – is moving away from us at a speed of 67.4 km per second. The second method keeps it at 73.3 km per second.
In today’s science of accuracy, you can no longer, unfortunately, cover up a 10% difference with the mantle of love. Is there something wrong with our ΛCDM form? Do you need different physics to describe the childhood of the universe? Many cosmologists secretly hope for this. Any trace of the blind spot in our theories about reality raises the hope that we can dig another layer deeper into our understanding of the universe.
But that doesn’t include Wendy Friedman. This astronomer, a professor at the University of Chicago, believes we’ve been working with faulty landmarks all along. Cepheids are usually embedded in dust clouds in star-forming regions, which interferes with their observation.
Could be better. Friedman searched and found even better landmarks: stars at the top of the red giant branch, a stage of evolution where all stars reach the same absolute brightness. “The advantage of this method is its simplicity: we understand these stars very well, you find them in all galaxies and they shine brightly,” says Friedman.
And imagine what? They provide a corresponding value for NS0 At 69.8 km per second per megaparsec, it is not statistically significantly different from the 67.4 km from the first method. Hubble tension gone? Friedman remains adamant: “We need to measure a lot of these stars to be sure enough.”
In the meantime, though, hopes for new physics in the early universe are fading.