Space
Can you really see the Great Wall of China from space?
Adjective: second.
Adjective: alien.
Adjective: different.
Adjective: Left, as opposed to right.
Noun: An other, another (person, etc), more often rendered as another.
Noun: The other one; the second of two.
Determiner: Not the one or ones previously referred to.
Adverb: otherwise.
Verb: or treat as an “other”, as not part of the same group; to view as different and alien.
Verb: To treat as different or separate; segregate; ostracise.
Can you really see the Great Wall of China from space?
You’ve probably heard the story that the Apollo astronauts could see the Great Wall of China from the moon. This claim sounds funny from a scientific point of view, but why can’t it be true?
The important issue is resolution. Astronomers usually use this term for the ability to separate two space objects that are at a close distance from each other, and sometimes you may see them as a merged point. You probably had this experience too: while driving at night on the highway, you see the lights of the cars as a single light source until the car approaches you and you realize that you are actually seeing two lights.
Resolution is usually expressed in terms of an angle, which is measured based on the size of the object and its distance from the observer. For example, the diameter of the moon is approximately 3,500 km and it is 380,000 km away from the Earth; By doing calculations, you will find that the moon has an apparent width of 0.5 degrees.
The average human eye has a resolution of approximately one minute of arc (there are 60 minutes of arc in one degree). A person with very strong vision can distinguish an object half this size. The moon is bigger than this example, so it is seen as a disk, and its features can be easily seen.
But what about the Great Wall of China? This wall is very narrow despite its long length. One of the widest parts of this wall reaches 10 meters. Can something this big be seen from the moon?
The Great Wall of China appears from the moon as a line with a thickness of one-thousandth of an arc minute. Recognizing such a structure from the moon is like trying to see a strand of human hair from a distance of one kilometer with the naked eye; So you probably agree that it is impossible to see the Great Wall of China from the moon.
A view of the Great Wall of China as seen by ESA astronaut Alexander Chert from the International Space Station in 2018. This image was recorded with an 800 mm super-telephoto lens.
However, let’s manipulate the above claim a bit: Suppose we are in low-Earth orbit and we are seeing our planet from the International Space Station (ISS). This station is located 400 km above the Earth’s surface, so the Great Wall of China appears from there with a width of 0.1 arc minutes. Even so, it is too small to see with the naked eye.
However, there is still a chance. The human eye can well recognize narrow and long objects that have a high contrast with their surroundings. For example, astronauts stationed in Earth orbit can see roads in the middle of the desert and traces of ocean-going ships. Does this observation help? Unfortunately no. The Great Wall of China is made of stones that do not contrast well with the ground and also have a curved shape depending on the landscape, especially near cliffs and hills.
NASA astronaut says he saw the Egyptian pyramids from space
We also know this empirically: astronauts tried to see the Great Wall of China but never identified it for sure. Orbital images that show detail were captured using telephoto lenses, which have much higher resolution than the human eye. Even Yang Livi, the first Chinese astronaut, said that he could not see the Great Wall of China from orbit, and this proud national structure definitely motivated him to at least try.
Of course, under certain conditions, the Chinese wall can be seen from orbit. At sunrise or sunset, when the sun is at the lowest point in the sky, the Chinese wall can create a long shadow; But again, this is not the same as seeing the wall itself.
But what can be said about other man-made objects? The Great Wall of China is long, but there are definitely other bigger structures. Egyptian pyramids are other obvious candidates. The Great Pyramid of Giza has an approximate diameter of 230 meters at the base, and thus it is large enough to be seen from low Earth orbit.
Earth city lights from the International Space Station
On the one hand, the dusty pyramid of Giza does not have much contrast with the sand around it, but the contrast can be reached from the light of the structure itself: in the low angles of the sun, half of the pyramid is lit and the other half is in the shadow, thus distinguishing it from a sandy landscape. It becomes possible. Leroy Xiao, a former NASA astronaut, claims to have seen two Egyptian pyramids and tried to see the other one, but failed.
According to NASA, the Three Gorges Dam in China can be seen from space. So far, there have been no reports of astronauts actually seeing it, but it is possible. This dam is huge. It is more than 100 meters wide at the base and its length reaches 2.3 kilometers. The light brown color of this dam appears next to the blue waters of the Yangtze River in the telephoto images of the astronauts.
It should be noted that you don’t necessarily need to separate an object to see it. Stars are good examples. Although they are very large and the diameter of some of them reaches millions of kilometers, they are remarkably far away; Even the nearest star (except the sun) is 40 trillion kilometers away from us. If there is enough light, even an object with low resolution can be seen.
For this reason, city lights are easily seen from space and are even visible from the moon under certain conditions. One of the problems here is that the bright environment of the moon can spoil this view. So the astronauts had trouble seeing the stars from the surface of the moon even when the sky was completely black. Also, the earth in the moon’s sky is much brighter than the moon’s view from the earth, and this feature disturbs seeing the earth’s lights.
Perhaps if a lunar astronaut is hidden in the shadow of a large boulder and the Earth is in a narrow crescent phase, cities can be seen very dimly on the night side of planet Earth. Perhaps future astronauts on the moon will be able to report this observation.
Read more: Hubble Space Telescope; A portal to the mysterious depths of the universe
Now let’s reverse the question: Can you see the International Space Station from Earth? Its light is definitely clear and sometimes it can even be brighter than Venus. The answer to this question is positive. The International Space Station is 100 meters wide, and when it is directly overhead, or in other words at its closest distance to Earth, it appears to be slightly less than a minute of arc in size; So a person with strong vision can see it as little more than a dot. Even with simple binoculars, you can see the space station as a branching line.
Finally, although man-made objects can hardly be seen from space, our impact on planet Earth is quite obvious. Canadian astronaut Chris Hadfield writes in his book The Astronaut’s Guide to Life on Earth that from the International Space Station he could see the effects of the destruction of Madagascar’s forests and the introduction of soil into the ocean. Forest fires due to climate change can be easily seen from space, and of course, the glow of cities is quite obvious from a distance of several hundred kilometers.
We are small in space, but we have left a big impression.
How the James Webb Space Telescope deepens cosmology’s biggest controversy
Summary of the article:
- Almost a century ago, Edwin Hubble discovered the expansion of the universe and calculated the expansion rate or the cosmic constant.
- Since Hubble, many groups have tried to measure the expansion rate of the universe. However, the values they obtained differed from the theoretical predictions. This difference is called Hubble tension.
- Scientists today use three methods to measure the expansion rate of the universe: Cephasian variable stars, TRGB red giant stars, and JAGB asymptotic giants.
- However, the Hubble tension still exists, indicating that the methods for calculating the Hubble constant suffer from a systematic flaw.
- Researchers hope to be able to use the James Webb telescope in the coming years to achieve more accurate measurements of the universe’s expansion rate and thus resolve the Hubble tension.
Almost a century ago, Edwin Hubble discovered that the universe was getting bigger. However, today’s measurements of how fast the universe is expanding are contradictory. These discrepancies show that our understanding of the laws of physics may be incomplete. On the other hand, everyone expected the sharp eyes of the James Webb telescope to bring us closer to the answer to the riddle; But a new analysis of the telescope’s long-awaited observations once again reflects inconsistent expansion rates from different types of data, while pointing to possible sources of error.
Two competing groups have led efforts to measure the rate of expansion of the universe, known as Hubble’s constant, or H0. A group led by Adam Reiss of Johns Hopkins University, relying on the known constituents of the universe and the governing equations, has consistently calculated the Hubble constant to be approximately 8 percent higher than the theory predicts the universe’s expansion rate. This discrepancy, known as the Hubble tension, indicates that the model of the cosmological theory may have missed some elements such as raw materials or effects that speed up the expansion of the universe. Such an element can be a clue to a more complete understanding of the world.
This spring, Reiss and his team published new measurements of the Hubble constant based on data from the James Webb Telescope and found a value consistent with their previous estimates. However, a rival group led by Wendy Friedman of the University of Chicago warns that more precise measurements are needed. The team’s measurements of the Hubble constant are closer to the theoretical estimate than Riess’ calculations, suggesting that the Hubble stress may not be real.
Since the commissioning of the James Webb Telescope in 2022, the astrophysical community has been waiting for Friedman’s multidimensional analysis based on telescope observations of three types of stars. The results are now as follows: the two-star types provide estimates of the Hubble constant that are in line with the theoretical prediction; While the results of the third star, which is the same type used by Reiss, are consistent with his team’s higher estimates of Hubble’s constant. According to Friedman, the fact that the results of the three methods are contradictory does not mean that there are unknown physical foundations, but that there are some systematic errors in the calculation methods.
Contradictory world
The difficult part of measuring cosmic expansion is measuring the distance of space objects. In 1912, American astronomer Henrietta Levitt first used pulsating stars known as Cephasian variables to calculate distances. These stars flicker at a rate proportional to their intrinsic luminosity. By understanding the luminosity or radiant power of a Cephasian variable, we can compare it with its apparent brightness or dimming to estimate its galaxy’s distance from us.
Edwin Hubble used Levitt’s method to measure the distances to a set of galaxies hosting the Cephasian variable, and in 1929 he noticed that the galaxies that are farther away from us are moving away faster. This finding meant the expansion of the universe. Hubble calculated the expansion rate to be a constant value of 500 km/s per megaparsec. In other words, two galaxies that are 1 megaparsec or approximately 3.2 million light years apart are moving away from each other at a speed of 500 km/s.
As progress was made in calibrating the relationship between the pulsation frequency of Cepheids and their luminosity, measurements of the Hubble constant improved. However, since the Cephasian variables are very bright, the whole approach used has limitations. Scientists need a new way to measure the distance of galaxies from each other in the infinite space.
In the 1970s, researchers used Cephasian variables to measure the distance to bright supernovae, and in this way they achieved more accurate measurements of the Hubble constant. At that time, as now, two research groups undertook the measurements, and using supernovae and Cephasian variable stars, they achieved contradictory values of 50 km/s per megaparsec and 100 km/s per megaparsec. However, no agreement was reached and everything became completely bipolar.
Edwin Hubble, the American astronomer who discovered the expansion of the universe, stands next to the Schmidt telescope at the Palomar Observatory in this photo from 1949.
The launch of the Hubble Space Telescope in 1990 gave astronomers a new and multi-layered view of the universe. Friedman led a multi-year observing campaign with Hubble, and in 2001 he and his colleagues estimated the expansion rate to be 72 km/s/Mpa with an uncertainty of at most 10%.
A Nobel laureate for the discovery of dark energy, Reiss got into the expansion game a few years later. In 2011, his group found the Hubble constant to be 73 with a three percent uncertainty. Soon after this, cosmologists excelled in another way. In 2013, they used Planck’s observations of light left over from the early universe to determine the exact shape and composition of the early universe.
In the next step, the researchers connected their findings to Einstein’s theory of general relativity and developed a theoretical model to predict the current state of the universe, up to approximately 14 billion years into the future. Based on these calculations, the universe should be expanding at an approximate rate of 67.4 km/s per megaparsec with an uncertainty of less than one percent.
Reese’s team measurement remained at 73, even with the improved accuracy. This higher value indicates that the galaxies today are moving away from each other at a faster rate than theoretically expected. This is how the Hubble tension was born. According to Reiss, today’s Hubble tension shows us that something is missing in the cosmological model.
The missing factor could be the first new element in the universe to be discovered since dark energy. Theorists still have doubts about the identity of this agent. Perhaps this force is some kind of repulsive energy that lasted for a short time in the early universe, or perhaps it is the primordial magnetic fields created during the Big Bang, or perhaps what is being missed is more about ourselves than the universe.
Ways of seeing
Some cosmologists, including Friedman, suspected that unknown errors were to blame for Hubble’s tension. For example, Cephasian variable stars are located in the disks of younger galaxies in regions full of stars, dust, and gas. Even with Hubble’s fine resolution, you don’t see a single Cephasian variable, according to George Afstatio, an astrophysicist at the University of Cambridge. Rather, you see it overlapping with other stars. This density of stars makes measurements of brightness difficult.
When the James Webb Telescope launches in 2021, Reiss and his colleagues will use its powerful infrared camera to peer into the crowded regions that host the Cephasian variables. They wanted to know whether the claims of Friedman and other researchers about the effect of the area’s crowding on the observations were correct.
The 6.5-meter multi-section mirror of the James Webb Space Telescope at NASA’s Goddard Space Flight Center in Maryland. This mirror passed various test stages in 2017.
When the researchers compared the new numbers to distances calculated from Hubble data, they saw a surprising match. The latest results from the James Webb telescope confirmed the Hubble constant measured by the Hubble telescope a few years ago: 73 km/s/Mpa with a difference of one kilometer or so.
Concerned about crowding, Friedman turned to alternative stars that could serve as distance indicators. These stars are found in the outer reaches of galaxies and away from the crowd. One of those stars belongs to the group ” Red Giant Branch ” or TRGB for short. A red giant is an old star with a puffy atmosphere that shines brightly in the red light spectrum. As a red giant ages, it eventually burns helium in its core, and at this point, the star’s temperature and brightness suddenly decrease.
A typical galaxy has many red giants. If you plot the brightness of these stars against their temperature, you reach a point where the brightness drops off. The star population before this brightness drop is a good distance indicator; Because in each galaxy, such a population has a similar distribution of luminosity. By comparing the brightness of these star populations, astronomers can estimate their relative distances.
The Hubble tension shows that the standard model of the cosmos is missing something
Regardless of the method used, physicists must calculate the absolute distance of at least one galaxy as a reference point in order to calibrate the entire scale. Using TRGB as a distance index is more complicated than using Kyphousian variables. MacKinnon and colleagues used nine wavelength filters from the James Webb telescope to understand how brightness relates to their color.
Astronomers are also looking for a new indicator: carbon-rich stars that belong to a group known as the “Jay region asymptotic giant” (JAGB). These stars are far from the bright disk of the galaxy and emit a lot of infrared light. However, it was not possible to observe them at long distances until James Webb’s launch.
Friedman and his team have applied for observation time with the James Webb Space Telescope in order to observe TRGBs and JAGBs, along with more fixed spacing indices and Cephasian variables, in 11 galaxies.
The vanishing solution
On March 13, 2024, Friedman, Lee, and the rest of the team meet in Chicago to find out what they’ve been hiding from each other. Over the past months, they were divided into three groups, each tasked with measuring distances to 11 galaxies using one of three methods: Cephasian variable stars, TRGBs, and JAGBs.
These galaxies also host related types of supernovae, so their distances can calibrate the distances of supernovae in many more distant galaxies. The rate at which galaxies move away from us divided by their distance gives the value of the Hubble constant.
Wendy Friedman at the University of Chicago is trying to fit the James Webb Telescope observations into the Standard Cosmological Model.
Three groups of researchers calculated distance measures with a unique, random counterbalancing value added to the data. During the face-to-face session, they removed those values and compared the results.
All three methods obtained similar distances with three percent uncertainty. Finally, the group calculated three values of Hubble’s constant for each distance index. All values were within the theoretical prediction range of 67.4. Therefore, Hubble’s tension seemed to be resolved. However, they ran into problems with further analysis to write the results.
The JAGB analysis was good, But the other two were wrong. The team found that there were large error bars in the TRGB measurements. They tried to minimize the errors by including more TRGBs; But when they started doing this, they found that the distance to the galaxies was less than they first thought. This change caused the value of Hubble’s constant to increase.
Friedman’s team also discovered an error in Cephaus’s analysis: in almost half of the pulsating stars, the term crowd was applied twice. Correcting this error increased the value of Hubble’s constant significantly. Hubble’s tension was revived.
Finally, after efforts to fix the errors, the researchers’ paper presents three distinct values of Hubble’s constant. The JAGB measurement yielded a result of 67.96 km/s/megaparsec. The TRGB result was equal to 69.85 with similar error margins. Hubble’s constant was obtained at a higher value of 72.05 in the Kyphousian variable method. In this way, different hypotheses about the characteristics of these stars caused Hubble’s stress value to vary from 69 to 73.
By combining the aforementioned methods and uncertainties, the average Hubble stress value equal to 69.96 was obtained with an uncertainty of four percent. This margin of error overlaps with the theoretical prediction of the expansion rate of the universe, as well as the higher value of Tim Reiss.
Tensions and resolutions
The James Webb Space Telescope has provided methods for measuring the Hubble constant. The idea is simple: closer galaxies look more massive; Because you can make out some of their stars, while more distant galaxies have a more uniform appearance.
A method called gravitational convergence is more promising. A massive galaxy cluster acts like a magnifying glass, bending and magnifying the image of a background object, creating multiple images of the background object when its light takes different paths.
Brenda Fry, an astronomer at the University of Arizona, is leading a program to observe seven clusters with the James Webb Space Telescope. Looking at the first images they captured last year of the G165 cluster, Fry and his colleagues noticed three spots that were not previously seen in the images. These three points were actually separate images of a supernova that was located in the background of the aforementioned cluster.
After repeating the observation several times, the researchers calculated the difference between the arrival times of the three gravitational lensing images of the supernova. This time delay is proportional to Hubble’s constant and can be used to calculate this value. The group obtained an expansion velocity of 75.4 km/s/Mpa with a large uncertainty of 8.1%. Fry expects the error bars to correct after several years of similar measurements.
Both Friedman’s and Reiss’ teams predict that they will be able to get a better answer with James Webb’s observations in the coming years. “With improved data, the Hubble tension will eventually be resolved, and I think we’ll get to the bottom of it very quickly,” Friedman says.
James Webb vs. Hubble; Comparing the images of two space telescopes
The James Webb Space Telescope is now the flagship of the world of space telescopes. James Webb uses the largest mirror of any telescope launched into space and orbits the Sun at a distance from Earth (approximately 1.5 million kilometers). This telescope is supposed to make a big change in astronomy and astrophysics by looking at the farthest parts of the world and unveiling new secrets.
However, in the past three decades, the Hubble Space Telescope has been the main player in the field of space telescopes. Launched in 1990, this telescope has recorded many epic images of planets, stars, galaxies, nebulae and many other astronomical wonders.
Since Hubble was in near-Earth orbit, it could take accurate pictures of the universe without the Earth’s atmosphere interfering with its operation. The telescope captured amazing images during its prolific career and continues to serve its mission. According to NASA, over the past 32 years, Hubble has made 1.5 million observations and contributed to more than 19,000 scientific papers.
Now James Webb is going to take the flag from Hubble and further enhance our understanding of the universe. In a blog post comparing the two space telescopes, NASA called James Webb the “scientific successor” to Hubble. Without Hubble’s observations of the cosmos, researchers would not have prioritized building a telescope that could go beyond Hubble’s field of view. Continue with Zoomit to have a closer look at the images of celestial targets in both telescopes.
Deep field
The top right photo is the deep Hubble wallpaper taken in 1995 and released in 1996. At the time of recording, this photo was considered the deepest image taken of the universe. In order to make it, the researchers took 342 photos during 10 days with a total of 100 hours of exposure. The final result revealed more than three thousand scattered galaxies in a very small part of the sky. Over the next few decades, Hubble operators took better pictures of this type and looked much deeper into space.
The Hubble Ultra Deep Image or XDF was released in 2012.
The photo above is the Hubble Ultra Deep Wallpaper released in 2012. More than 5500 galaxies are visible in this photo. Over the course of a decade, the researchers collected 50 days of observations from a concentrated area, resulting in two million seconds of exposure (over 23 days).
Then along came James Webb. The first full-color scientific photo by James Webb was released on Tuesday morning Iran time by US President Joe Biden as a preview for the first set of telescope images. While capturing Hubble’s deep fields required days, if not weeks, of exposure, James Webb managed to capture this image after just 12.5 hours of exposure.
The recorded area of the sky in the image above is incredibly small; So small that it is about the size of a grain of sand on a person’s hand on the ground. In that part of the sky, a galaxy cluster called SMACS 0732 is located 4.6 billion light-years away. This cluster is so large that it bends space-time around it and, like a cosmic microscope, reveals faint galaxies far behind it. Some of these galaxies are considered to be the faintest infrared objects ever observed, and scientists are eagerly waiting to learn more about them.
Shahabhi Nebula (Karina)
The image above shows one of Hubble’s most popular targets, the Carina Nebula, at a distance of 7,200 light years from us. The image on the right, released by the Hubble Heritage Project in 2008, shows part of the star-forming region in the corner of the nebula.
The picture looks like an impressionist landscape with hills, valleys, and columns of gas and dust scattered around, with only a few bright stars behind the nebula. Now James Webb’s updated image shows the same stunning landscape in much greater detail and clarity. In the photo, there are stars that were previously hidden behind gas and dust.
Read more: Why is Jupiter one of the first targets of the James Webb Space Telescope?
Stephan’s quintuplets
This group of five galaxies is stunning in a Hubble image taken in 2009 after the telescope’s camera was upgraded earlier that year. That year, the space shuttle made its fifth and final visit to Hubble and applied a major upgrade. In addition to the new camera that took this photo, the telescope was also upgraded and repaired. The ability of astronauts to rendezvous with Hubble in near-Earth orbit kept the space telescope operational for a very long time.
However since James Webb is so far from Earth, he will not have the advantage of meeting astronauts. However, the spacecraft has enough fuel for at least 20 years; This means that in the future we will see many more images like the one above.
The photo above shows a group of galaxies that were first observed in 1877. The upper left galaxy is considered the alien mass of the group and is much closer to Earth than the other four members. However, the other four galaxies are so close that James Webb can see the shock waves from the interaction between them as they kill each other.
Stephen’s quintuplet is the largest image taken by the James Webb Telescope and is actually a mosaic made up of more than a thousand separate images captured by the telescope’s two instruments: the Near Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI). Both cameras collect infrared wavelengths of light and help James Webb see through gas and dust; But as their names suggest, both collect different wavelengths of infrared light.
Southern Ring Nebula
The last image is the Southern Ring Nebula, which was imaged by Hubble in 1998. The “ring” is the dying star particles (the faintest of the two bright spots in the center of the image). The little white dwarf causing all this chaos was a star the size of our Sun. At some point, the star ran out of fuel and ejected its outer layers, creating the ring seen in the image. The diameter of the ring is about half a light year and the gases are moving outward at a speed of nearly 14.5 km/s.
On the left, the Southern Ring Nebula is seen through the eyes of James Webb’s two instruments, Nirkem and Miri. The image we have chosen for comparison was captured by Nirkam’s near-infrared camera. Another version of the image is also recorded in mid-infrared.
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