Sabine
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Sunday Special: What may the Webb telescope discover?
🕑 Added 2022-01-30 17:12:39 +0000 UTCThe James Webb Space Telescope has finally launched. And that’s exciting not just for astrophysicists, but for all space lovers. What makes the Webb telescope new and different? What is it looking for and what may it discover? That’s what we will talk about today.
The James Webb Space Telescope is a joint project between NASA, ESA, and the Canadian Space Agency. I’ve heard astrophysicists going on about it since I was a student.
In 1999, the Webb telescope was scheduled for launch in 2007, but in 2003 a redesign pushed the date to 2010.
In 2005 it turned out that the cost estimate didn’t pan out. The whole thing was replanned and the launch date moved to 2013. And then to 2015. And then to 2018. And then some further delays made that 2020. And then COVID happened. It was finally launched on Christmas day 2021 after more than 3 decades of planning.
Now everyone hopes the wait was worth it. The James Webb telescope is an infrared telescope. It’s equipped with four different instruments that can detect light in the wavelength regime from zero point six to 28 micrometers. So that is at the long wave-length end of visible light, and then below that.
The previous infrared telescope was the Spitzer Telescope. But it required liquid helium for cooling and that ran out in 2009. After that, the Spitzer telescope operated with reduced functionality until it was retired in 2020. The James Webb telescope will have a better resolution than Spitzer. Here is a simulation of how much better. On the left is the Spitzer resolution on the right what we expect from Webb in comparison. Indeed, a group of astrophysicists have done a simulation of a far field image from Webb into which you can zoom and zoom and zoom. Link to their paper and website is in the info below.
What’s so great about infrared light? Well, each wave-length range is good for something else. Infrared light in particular is good to see through dust. And space is full of dust. Dust is made of small particles, and often they are of a size that’s about the same as the wave-length of visible light. This means visible light scatters a lot on dust. Infrared light is scattered far less because of the longer wave-length, so one can use it to see through the dust.
This is interesting for example because a lot of galaxies or galaxy clusters are surrounded by dust so we don’t really know what’s going on inside. Here are example images from the Hubble space telescope, on the left, in the visible range. And from the Spitzer infrared telescope on the right. Look how the dust has basically disappeared. Now you can see inside. The James Webb telescope can do that too, but at higher resolution. And compared to Hubble, Webb will have a larger field of view covering more thn 15 times the area that Hubble covers.
Another reason why infrared is great is that as the universe expands, wavelengths stretch. So the light from early stars and galaxies becomes shifted to the red. This means, the better you can measure on the red end, the more you can see of the early stars and galaxies.
That Webb is an infrared telescope is also the reason why the mirror is gold. Gold is not the greatest reflector in the visible range, but it is a great reflector in the infrared. It’s a tiny amount of gold that’s been used for those mirrors, because the cover is only about 100 nanometers thick. In total that’s less thn 50 grams of gold.
That other big piece on the telescope that’s the sun shield. It’s there to keep the instruments cool. Webb has four different instruments, each of which can detect somewhat different properties of the light that the telescope collects. By the end of January, the Webb telescope reached its final position which is the Lagrange point two of the sun-earth system, that’s farther away from the sun than we are. The good thing about the Lagrange point is that the telescope can orbit around it with only small corrections, so it won’t need a lot of fuel. The fuel supply for the propulsion system is designed to last for about 10 years, but maybe in 10 years it’ll be possible to refill it.
The sunshield will block emissions from both the sun and also the earth. The temperature differences between the two sides are remarkable. One the side facing towards the sun it’s as high as 80 degrees Celsius and on the side facing out into space, just 40 degrees above absolute zero. Still, for one of the instruments on board the Webb telescope that isn’t cold enough, it needs to be cooled to 7 degrees above absolute zero. Webb does that with a cooling system that won’t just exhaust the supply of a cooling agent, but that can keep on going as long as the equipment doesn’t fail and as long it has power. Where does the power come from? It comes from the sun. Lots of solar power out there in space.
So what do astrophysicists want to do with the Webb telescope? Well the way that it works in astrophysics is that you apply for time with a telescope so you can collect the data you want. The Webb telescope will be used by those groups whose research proposals have been accepted. And the people who worked on the telescope development have some time reserved for themselves. You can look up on the website which research programs have been accepted. Here is an overview on the topics. I will leave you a link in the info below so you can look at the complete list yourself. The observation cycles will each go about a year. The first cycle will start after Webb has completed checks, probably around June this year.
As you can see, the scientific program covers quite a variety of topics. One is scanning exoplanets for signs of molecules that might signal the presence of life. Infrared light is good for that because the absorption signatures of molecules like oxygen, water, carbon dioxide, ozone, and methane are in that range.
Another big bunch of topics is the formation of stars and solar systems that is often obscured in other wave-length ranges. From Webb we might learn a lot about how all that dust manages to clump together and form planets. There is also the dust itself. I know I said that you can use infrared light to see through dust, but at somewhat shorter wavelengths you’ll also begin to test the chemical make-up of dust clouds. And some researchers want to use the Webb telescope to look at objects inside the solar system. For example at asteroids, to find out whether there’s water on them or something else. This would be very helpful to find out where the water on earth comes from which is somewhat of a mystery. And that again would be very helpful to understand how likely it is that life developed elsewhere in the universe.
All of this is pretty cool, but I am personally most excited about the observations on young galaxies, at extremely high redshifts, early in the universe. The youngest galaxy that the Hubble telescope has seen has been estimated to date back to abou(t) 400 million years after the big bang. Webb should be able to see back to about 100 million years after the big bang.
That’s very interesting because the way that galaxies form tells us something about the matter in the universe, in particular about dark matter and its role in structure formation. In the currently most widely accepted theory for cosmology, the large galaxies we see today build very gradually by merging smaller galaxies. This figure shows how astrophysicists think this works. All the symbols here are galaxies and the larger the symbol the larger the galaxy. Time increases from the bottom up. At the beginning you have all these tiny galaxies, and then they join to increasingly larger ones.
What you can see from this graph is that if this theory is correct there basically shouldn’t be any large galaxies at very early times. But is this correct? This figure shows the predictions from the millennium simulation in comparison to data. You can see two things here. One is that there isn’t a lot of data at the moment. But also that it seems like the data is way off the simulation.
The millennium simulation was a large computer simulation for structure formation in the standard model of cosmology. In such a simulation, you basically distribute dark matter in the early universe and then you let it clump following its own gravitational pull. Normal matter mostly follows the gravitational pull of the dark matter, but then the normal sticks together better and forms stars which dark matter doesn’t do, or at least isn’t expect to do.
The millennium simulation used about 10 billion particles to study structure formation. That was pretty amazing in 2005, but today computing power has much improved. The newest simulation for structure formation is the Uchuu simulation that was just released a few months ago. It contains about two trillion particles, or to be precise, 2,097,152,000,000. You can also download this data yourself if you want. At least If you have 125 Terabytes of free disk space. There will without doubt soon be papers coming out that quantify the structures they have found in this simulation.
They will probably confirm the findings of the earlier simulation, namely that galaxy formation with dark matter takes a long time. You don’t expect to see large galaxies early on in the universe if the standard model of cosmology is right.
If the Webb telescope sees large galaxies anyhow, then that’s going to be very difficult to explain with dark matter. That, in my opinion would be the most interesting discovery the telescope could make. Though, I guess oxygen and water on an exoplanet would be a close second. What do you hope Webb will see? Let me know in the comments.
Comments
Sabine: You gave an interesting information here about dark matter: “Normal matter mostly follows the gravitational pull of the dark matter, but then the normal sticks together better and forms stars which dark matter doesn’t do, or at least isn’t expect to do.” This is another hint that photons having a strong gravitational field may build the dark matter. Because they will support structure formation but are of course not part of the object built by it. The idea of photons with a gravitational field are not as strange as it may sound. In 1911 Einstein was looking for new ideas about gravity. But in his calculation regarding the variation of c in a field he made a mistake. He did not continue this way. If that original way is continued – without the mistake – it ends quite easily at a theory of gravity where photons play an essential role. And this explains all observations of dark matter and is in conflict with no one.
I wish that it wouldn't take so long to get set up for viewing, but June will be here soon enough. The images will be worth the wait as Hubble's were in its day.
