We all know that the universe is vast. But when we look in either direction, the farthest visible region of the universe is about 46 billion light-years away. But this is actually our best estimate, because no one knows for sure how big the universe is.
The furthest distances we can see are the distances light has traveled since the Big Bang (or, more accurately, the microwave radiation ejected from the Big Bang). About 13.8 billion years ago, the universe was born in a big bang, and it has been expanding ever since. But since we don't know the true age of the universe, it's hard to determine just how far the universe has expanded beyond what we can't see.
Astronomers have tried to use the "Hubble constant" to determine how much the universe is expanding. This is a measure of the current rate of expansion of the universe, and the Hubble constant determines the scale of the universe, including its size and age.
We might as well compare the universe to an expanding balloon. As stars and galaxies (like the blobs on the surface of a balloon) move away from each other faster and faster, so do the distances between them. From our perspective, the farther a galaxy is from us, the faster it dims.
Unfortunately, the more astronomers measure the Hubble constant, the less tenable the predictions we make based on our understanding of the universe. One measurement directly gives us a certain value, while another (depending on our understanding of other parameters of the universe) gives different results. Either both measurements are wrong, or our understanding of the universe is flawed.
But now, scientists believe, they are not far from the answer. Of course, all this is possible without new experiments and observations aimed at understanding the nature of the Hubble constant.
The challenge as a cosmologist is actually an engineering one: How can we measure this constant as precisely and accurately as possible? Addressing this challenge requires not only obtaining the measured data, but also cross-checking the measurement method in as many ways as possible. From a scientist's point of view, it's more like putting the puzzle together in its entirety, rather than solving it.
Astronomer Edwin Hubble made the first measurement of the Hubble constant in 1929, which was named after Edwin Hubble. The first measurements set the Hubble constant at 500 (km/s)/Mpc, or 310 (miles/s)/Mpc. Mpc stands for megaparsecs, a cosmic distance scale, roughly equivalent to a distance of 3.26 million light-years. 500 (km/s)/Mpc, which means that for every megaparsec the distance from Earth increases, the speed of the galaxy moving away from us increases by 500 km/s.
In the more than a century since Hubble first estimated the expansion rate of the universe, that number has been revised downward again and again. Today the value of the Hubble constant is between 67(km/s)/Mpc and 74(km/s)/Mpc. Part of the reason is that the Hubble constant varies depending on how you measure it.
Most explanations for the difference in the Hubble constant hold that there are two ways to measure the value of the Hubble constant. The first method looks at the speed at which galaxies near the Milky Way are moving away from us, while the other method chooses to use the cosmic microwave background (the first light left after the Big Bang).
We can still observe the cosmic microwave background today. But as distant regions of the universe are moving further and further away from us, this light is stretched into radio waves. Astronomers first discovered these radio signals by chance in the 1960s. These radio signals also give us a chance to understand what the universe looked like in its earliest days.
Two mutually repulsive forces, the inward pull of gravity and the outward push of radiation, staged a cosmic tug-of-war at the beginning of the universe, and the resulting disturbances still exist in the cosmic microwave background in the form of tiny temperature differences. middle.
From these perturbations, researchers can measure the rate at which the universe expanded shortly after the Big Bang, and then apply this to the Standard Model of cosmology to infer the current rate of expansion. This Standard Model is currently the best explanation for the origin of the universe, its composition, and everything we see today.
But there is a problem here. When astronomers tried to measure the Hubble constant using the first method, observing the speed at which galaxies near the Milky Way were moving away from us, they got a different value.
If the Standard Model is correct, then you would think that the results from the two approaches—the current measurements and those derived from earlier observations—should agree. However, it is not.
In 2014, the European Space Agency's Planck satellite measured the difference in the cosmic microwave background for the first time; in 2018, it measured it again. According to measurements by the Planck satellite, the value of the Hubble constant is 67.4 (km/s)/Mpc. However, this value is about 9 percent lower than what astronomers like Friedman had made by looking at nearby galaxies.
Further measurements of the cosmic microwave background by the Atacama Cosmology Telescope in 2020 correlate with data from the Planck satellite. This helped scientists rule out systemic problems with the Planck satellite in two ways. Then, if the measurement of the cosmic microwave background is correct, the remaining possibilities can only be one of two: (1) measuring the light from nearby galaxies, which is not right; (2) the standard model of cosmology requires Revise.
The measurements used by astronomers employ a special type of star: a Cepheid variable. About 100 years ago, astronomer Henrietta LeWitt discovered such stars that vary in brightness, with periods of days or weeks. LeWitt found that the brighter the star, the longer it takes to brighten, dim and then brighten again. Now, astronomers can accurately determine the true brightness of stars by studying the brightness pulses of such stars. By measuring the brightness we observe on Earth, coupled with the dimming of light with distance, we can accurately measure our distance from the star.
Friedman and her team were the first to use Cepheids in nearby galaxies to measure the Hubble constant. The data they used came from the Hubble Space Telescope. In 2001, they measured the Hubble constant to be 72 (km/s)/Mpc.
Since then, the value of the Hubble constant, derived from studying nearby galaxies, has fluctuated around 72 (km/s)/Mpc. Another team that also used Cepheids to measure the Hubble constant, using data from the Hubble Space Telescope in 2019, came up with a result of 74 (km/s)/Mpc. A few months later, another group of astrophysicists, using a different measurement technique involving light from quasars, came up with a value of 73 (km/s)/Mpc for the Hubble constant.
If these measurements are correct, this suggests that the universe may be expanding faster than the theory under the Standard Model of cosmology would allow. That said, the existing Standard Model — and the nature of the universe we describe based on that model — needs to be updated. For now, the answer is uncertain. But if it were, it would have profound implications for everything we know.
"It might tell us that something is missing from what we think of as the Standard Model," Friedman said. "We don't yet know why this is, but it's an opportunity to discover why."
If the Standard Model is wrong, then it could mean that some of our models—models about the composition of the universe, models of baryonic (or normal) matter, dark matter, the relative amounts of dark energy and radiation, etc., are not quite correct. Also, if the universe is indeed expanding faster than we thought, the universe may also be younger than the currently accepted 13.8 billion years old.
Another explanation for the difference in the value of the Hubble constant is that there is something different or special about the part of the universe we are in compared to the rest, and it is this difference that distorts the measurements. Maybe not a perfect analogy, but you can think of it this way, even if you hit the accelerator with the same amount of force when going uphill or downhill, the speed or acceleration of the car changes differently. This is unlikely to be an ultimate cause of the discrepancy in our measured values of the Hubble constant, and it is important that we cannot ignore the work done to arrive at these results.
But astronomers believe they are getting closer to determining the value of the Hubble constant and which measurement is correct.
"It's exciting, I think, that we can really solve this problem in a fairly short period of time, whether it's a year or two or three years," Friedman said. "There are a lot of new technologies that are coming out that can improve our Accuracy of measurements. Ultimately, questions can be answered."
One of these new technologies is the European Space Agency's Gaia space telescope. Launched in 2013, the Gaia space telescope has been measuring the positions of about a billion stars with high accuracy. Scientists are using a technique called "parallax" to calculate the distances between stars based on this data. As Gaia orbits the sun, the telescope's vantage point in space also changes. It's like if you cover one eye to see an object and then cover the other eye to see the object, the position of the object will look different. So, within an orbital period, Gaia can observe celestial objects at different times of the year, allowing scientists to accurately calculate the speed at which stars are moving away from our solar system.
Another device that can answer the value of Harper's constant is the James Webb Space Telescope. The telescope will launch in late 2021. The James Webb Space Telescope can make better measurements by studying infrared wavelengths. Such measurements would not be affected by the dust between us and the star.
But if these new techniques still find discrepancies in the value of the Hubble constant, then we do need to introduce new physics. Although many theories have been proposed to explain this difference, none can fully explain everything we see. Every potential theory has drawbacks. For example, it has been suggested that another type of radiation may have existed in the early universe, but we have precisely measured the cosmic microwave background, so the possibility is almost nil. Another view is that dark energy may change over time.
This seems like a very promising hypothesis, but for now, there may be other constraints on how dark energy changes over time. Dark energy only seems to change over time in an unnatural way, and it seems hopeless. Another explanation is that dark energy existed in the early universe and then disappeared. However, we have no apparent reason why dark energy exists in the first place and then disappears.
So scientists have to keep exploring new possibilities to explain what's happening right now. While we don't know what a plausible explanation is right now, that doesn't mean there won't be suitable ideas in the future.
.jpeg)
