#cosmic microwave background

[doodles-with-noodles]

[oliviabutsmart]

Physics Friday #16: The Cosmic Microwave Background (CMB)

This was originally going to be a post on Baryon Acoustic Oscillations, but I think that it's better to first discuss the CMB broadly before getting onto that, keeping the post lengths short(er).

Introduction: Our bath of Microwave Radiation

The CMB is everywhere, literally. Where you stand right now you are being bombarded by a bath of microwave radiation originating from the CMB. In fact, it's part of what causes the static on old TVs.

Now don't worry, the radiation is very weak, so it's unlikely you'll actually be hurt.

The CMB ceased to exist about 13 billion years ago, what you see in the microwave part of the spectrum is just what was left over.

Because the speed of light is not infinite, when we look really far out in the distance, we actually look back in time.

Measuring an object 1 light-year away means we only see what that object looks like a year ago. Measuring an object 1 billion light-years away means we are looking at that object 1 billion years in the past (ignoring the expansion of the universe).

If we account for an expanding universe, light actually travels a lot further. Because what was 13 billion ly has expanded into ~40 billion ly. However, we still see it as 13 billion ly.

This allows us to see the universe in it's early stages of life.

Eras of the Early Universe

The eras of our universe are crucial to understanding the CMB. In the first few seconds after the big bang, we have the hadron era, where the first protons and neutrons form.

A few minutes later, the temperature of the universe drops enought that these protons and neutrons then form into stable atomic nuclei during what's known as big bang nucleosynthesis.

Effectively we have nuclear fusion happening everywhere in space, like in the core of most stars, including our sun.

Eventually our universe expands to such a point that the temperature drops below a point where we cannot have nuclear fusion. But the temperature is still high enough that the universe is exclusively plasma.

Plasma is the state of matter in which electrons cannot bind themselves to their respective atomic nuclei, the universe is considered 'ionised' - as it's all just electricity.

During this time, the universe is opaque. Because any light moving around in the universe gets instantly absorbed by the loose electrons or nucleons.

We'll have to wait a couple thousand years before the universe cools even further, to the point where we get normal gaseous matter. The universe suddenly becomes translucent. This is known as the era of recombination.

At the moment of this occurring, any light emitted by the previously hot plasma escapes and can travel freely through the universe unobstructed. This thermal radiation comes in the form of infrared radiation.

Travelling Radiation

The radiation of the CMB is the earliest possible light we can see in the universe. Because any previously-existing light is obscured by the plasma earlier-on.

The radiation emitted from the CMB travels across space in order to reach us. And during that time it gets 'corrupted' in a sense.

Redshift - Turning Infrared into Microwaves

Redshift is a phenomena where light waves will stretch, their wavelength is increased as they are released and move through space.

Expanding space means that the CMB is effectively moving away from us at very fast speeds. This creates a doppler effect where the light waves will inherently be released out towards us at a longer wavelength. This is called doppler redshift.

The expansion of space also affects the wavelength as the photon travels. The space in-between each photon's wavelength stretches too, creating more cosmological redshift.

For most objects, cosmological redshift is negligible, because the light doesn't travel far enough for space to expand inside the wave. But the CMB is very far away, meaning that both forms of redshift are non-negligible.

Image credit: Forbes

The CMB exists at a redshift of ~1100, this doesn't mean much on it's own. What it means is that red-coloured light will become shifted to short radio frequencies that you can pick up on your radio.

The Lyman-Alpha Forest

Of course, our translucent gas is not perfectly translucent. Gas can absorb and emit specific wavelengths of light that perfectly correspond to energy levels an electron can take within an atom.

The Lyman series is an example of hydrogen emission/absorption lines, and as the light travels through hydrogen gas, the wavelengths corresponding to the Lyman series gets absorbed by our atoms.

This process keeps happening. As redshift alters the wavelengths of the surviving waves, any gas obstructing the CMB's light will have fresh light to absorb.

The effect is this continual series of absorption lines appearing in the otherwise clear spectra of radiation. Creating a 'forest' of several absorption lines.

Image Credit: Wikipedia

Re-Ionisation

Some gas in the early universe ends up converting back into plasma, this is caused by gravity pulling on small inconsistencies in the gas density.

These bright spots of plasma were what formed the first stars, galaxies, and black holes. But for us, it also effects the CMB.

Of course, ionised gas is opaque, so any light that happens to come across a star in-formation is absorbed instantly.

Observing the CMB

There have been multiple attempts to view the CMB. Despite all of our obstructions, space is mostly empty. Not just that, but the visible blemishes on our dark sky (like the milky way) can be removed in-post.

There are three notable missions: COBE (1989-93), WMAP (2001-10), and lastly the ESA Planck/BICEP mission (2015-19). Each increasing in resolution, each adding more information to our measurements.

When the CMB was first observed in the COBE mission, it saw something really strange, the CMB was really smooth, and looked like it had a dipole in it. This is the top image on the below diagram.

Image Credit: NASA

This dipole is actually caused by the orbital motion of the Earth! Not just around the sun, but also within our galaxy.

Alright, so let's try and re-configure our calculations, this time we will account for the movement of the Earth.

What we get is the middle image. There's still a big band in the centre of the frame, which actually is caused by our milky way, as the milky way itself also emits microwave radiation.

Thus this gives us the lower image. A background which looks approximately the same everywhere, but is distinctly not on local scales. This discovery broke one of our main assumptions in cosmology at the time - which is that the background is the same in all directions.

Something interesting also occurs as well. When all movement in space is removed, the CMB becomes static. This means that the CMB can act as a universal frame of reference, from which all other reference frames can be compared against.

This concept appears to violate the principle of relativity, but at the same time, our cosmological models require a reference frame to compare against. This is one example of an unsolved problem in GR.

While there isn't a visible dipole in our CMB observations, a dipole would have significant ramifications on our frame-of-refernce models.

Refining the Map and Using it

WMAP and the Planck mission refined our data to a point where we can use it for more interesting measurements.

Image Credit: ESA

The size of each small blob actually corresponds to the curvature inherent in the universe. As when we look far back into space, different curvatures will create different apparent sizes.

Not just that, but we can use the CMB to observe the reverberations of sound waves, this analysis allows us to measure the amount of matter that existed in the early universe.

With both measurements of curvature and matter density, we can estimate both the strength of dark energy and the expansion rate of the universe, allowing us another method to quantify dark matter.

These two data points turn out to be observed using the same phenomena, Baryon Acoustic Oscillations (BAOs) - a future topic (maybe in 2 weeks' time).

Conclusion

The CMB is still being investigated and still being observed. The main reason is because it is our only existing window into viewing the early universe.

Not just that, but the CMB provides important data that gave rise to both inflation theory and the eventual formation of our structure in the universe.

I will eventually cover inflation theory. But sooner, I will talk about BAOs. As they are also a rather interesting discussion in cosmology. The measurement of BAOs is what has led to our current cosmological crisis, where the CMB's measurements misalign with other data points in measuring the strength of dark energy.

As always, please feel free to comment your thoughts, including criticisms. Follow if you want to see more!

[lenbryant]

Baby pictures!

[religion-is-a-mental-illness]

At what point in the first six days did God create the Cosmic Microwave Background?

Probably when he was hiding all the fossils while he was waiting for his burrito to finish microwaving.

[planetariumhub]

Unveiling the Cosmic Remnant: Exploring the Crab Nebula (M1)

Credits: NASA, ESA, J. Hester and A. Loll (Arizona State University) Among the fascinating remnants of stellar explosions, the Crab Nebula, also known as Messier 1 (M1), stands as a testament to the immense forces that shape our universe. Located in the constellation Taurus, this celestial spectacle has captivated astronomers and enthusiasts alike for centuries. In this article, we embark on a…

[jmillz888]

Giving Stranger Things and Quantum Entanglement vibes :)

oh okay. heart steps right out of my chest and falls down the stairs

[g0fckur531f]

[slypuzzle]

Dark Matter: The Invisible Force of the Universe

Did you know that most of the universe is made up of invisible dark matter? Though we can’t see or touch it, dark matter holds galaxies together and shapes the cosmos!  

[sciencestyled]

The Accidental Astronomer: How Tutankhamun’s Quest for the Perfect Breakfast Led to Cosmic Revelations

In the golden age of Egypt, where the pyramids kissed the sky and the Nile nourished the land, I, Pharaoh Tutankhamun, embarked on an endeavor most peculiar. It began on a morning like any other, except, perhaps, for my insatiable craving for a breakfast not of this world. "Miu-Miu," I declared to my feline advisor, who lounged beside my golden goblet, "today, we seek a feast that rivals the gods!" Miu-Miu, whose wisdom was surpassed only by her appetite, nodded in solemn agreement, her whiskers twitching with anticipation.

Our journey led us beyond the palace walls, into the bustling markets of Thebes, and through the whispering reeds of the Nile. Yet, as the sun arched westward, our quest remained fruitless. Dejected, we returned to the palace, where a curious sight befell us. Amidst my royal chamber, a peculiar artifact lay untouched – a gift from a distant land, a device they called a "telescope."

In a twist of fate, driven by whimsy and perhaps a touch of divine madness, I aimed this celestial tube toward the heavens, hoping to catch a glimpse of a divine pantry. What I found, however, was not the secrets to an ethereal breakfast but a vista that stretched beyond the horizon of my understanding. The stars, the moon, the very fabric of the night sky unraveled before me, a cosmic feast for the eyes.

"Miu-Miu," I whispered, my voice trembling with awe, "the universe is far more vast and filled with wonders than any kingdom on earth." The stars twinkled in silent agreement, and in that moment, under the celestial dome, I felt a hunger of a different kind – a thirst for knowledge about the cosmos.

Night after night, Miu-Miu and I observed the heavens, recording our findings on papyrus scrolls. We noted the patterns of the stars, the phases of the moon, and the shifting tapestry of the night sky. Yet, it was a discovery made on a particularly clear night that would forever change the course of our cosmic inquiries.

As I peered through the telescope, a faint, omnipresent glow seemed to envelop the night sky. Puzzled, I consulted the wisest astronomers and seers in my kingdom, but none could explain this mysterious luminescence. It was then, in a dream visited upon me by Thoth, god of wisdom and knowledge, that I understood the true nature of my discovery.

Thoth revealed to me that this glow was not a trick of the eye nor a figment of my imagination, but the remnants of a cosmic banquet, the universe's own breakfast, if you will, from a time before time itself. This light, Thoth explained, was the Cosmic Microwave Background (CMB) – the afterglow of the universe's creation, a celestial feast spread across the entirety of the cosmos.

Armed with this divine insight, I resolved to share my newfound knowledge with the world. "Miu-Miu," I proclaimed, "we shall pen a guide to the stars, a manual that will illuminate the mysteries of the cosmos for all of humanity!" And so, with a heart full of wonder and a quill poised to scribe our celestial discoveries, I began to write the article that would serve as a beacon for star gazers and scholars alike: "Pharaoh’s Guide to the Stars: Tutankhamun Explains the Cosmic Microwave Background."

Thus, from a quest for a breakfast unlike any other, emerged a tale of cosmic proportions. A story that began with an unfulfilled appetite led to revelations about the universe's oldest light, guided by the paws of a feline and the wisdom of the gods. And while the secrets of the universe began to unfold before us, Miu-Miu and I often wondered – could the gods themselves enjoy breakfast as vast and mysterious as the cosmos? Perhaps, in our quest for knowledge, we had stumbled upon the divine feast we sought all along.

[adastra-sf]

this comes to mind:

- quote from the opening of William Gibson's Neuromancer

(arguably one of the best first lines, bursting with potential interpretations)

What you're seeing in old TV static is untuned radiation picked up by the receiver and displayed on the cathode-ray tube (a low-energy particle accelerator) coming from, among other sources, the cosmic microwave background radiation - remnants of the Big Bang.

And yes, you can touch that static, and it feels like prickly fur. Perhaps more symbolism or metaphor embedded in the line.

[playatsanity]

[oliviabutsmart]

Physics Friday #18: Sounds of the CMB - Baryon Acoustic Oscillations

Introduction: What Are BAOs?

It's time for colours!

Let's break down the term Baryon Acoustic Oscillations (BAOs) ...

A Hadron is a particle made of several quarks, a Baryon is a Hadron that contains an odd number of quarks.

Baryonic matter makes up a majority of the stable, visible matter in the universe. Both protons and neutrons are types of Baryons, so most Baryonic matter is, shockingly, atoms.

Acoustic generally has two definitions, one musicological and one scientific. Scientifically, any 'acoustic' thing involves sound. Specifically here, we're using it to discuss the production and propagation of sound.

Oscillations is a term very related to sound, if you read my posts on the mechanics of sound, oscillations, waves, periodic motion, are all the same phenomena.

Here, an acoustic oscillation is just a looping/repeated physical motion in a medium, like plasma.

Thus, putting everything together, a BAO is the physical motion in a plasma of hydrogen/helium, caused by the propagation of sound waves.

How do they form?

After the era of nucleosynthesis, but before the era of recombination, the universe was full of an opaque plasma of hydrogen/helium.

Sound waves require something that can create both a compression and expansion of this plasma, causing a pressure wave. A force that can pull things together and push things apart.

Thankfully, gravity is a great mechanism to pull things together. All that is required is a few thousand years, and some already pre-existing variations in density to get going.

So this plasma, in more dense regions, begin to pull more matter into those regions, creating a self-reinforcing cycle.

But how do we create the effect of expansion?

This plasma, is, well, hot. Very hot. And because it's everywhere, there's a lot of radiation being constantly emitted and re-absorbed.

When you begin to concentrate a lot of plasma into one location (thanks to gravity), you also concentrate the release of radiation.

Moreover, the speed the plasma accrues when it starts falling into the dense regions will also give it more energy, causing even more radiation to be released.

The effect is that in these denser clouds of gas, more radiation is produced. This produces a radiation pressure, which attempts to force atoms out of the dense regions.

Because of the every-where-ness of this plasma, the radiation pressure is strong enough to completely counteract gravity in some cases.

This generates sound waves, created by the constant tug-of-war between radiation pressure and gravity. These sound waves propagate throughout the early universe.

In Comes Dark Matter

Dark matter is predicted to represent a majority of the matter in our universe, because it is dark, it does not experience radiation pressure.

But because it still has mass, it does experience gravity. This means that the dark matter will stay within the denser regions without rebounding at all.

The dark matter within these regions are what allow regular matter to constantly fall in and out. Without it, the radiation pressure would blow apart the region and it'd become devoid of matter.

Freezing Things in Place

Suddenly, the universe cools to a point where, near simultaneously, matter cools to the point that it can exist as a gas rather than plasma.

Unlike plasma, gas does not absorb all forms of electromagnetic radiation. This is because electrons are now bound to their respective atoms.

This means that radiation pressure no longer comes into effect, and any radiation emitted at the moment of this cooling is free to travel across the universe.

The newly freed radiation is what we see in the CMB - but there's something important about it. It records a single snapshot of the current state of the sound waves propagating in the universe.

Imagine you have a high-speed camera and take a photo of a guitar string being plucked. At that very instant you can see the wave frozen in place, and can use that to determine it's properties.

Image Credit: Reddit

The remaining oscillations in the matter freeze too, allowing for the formation of our modern galaxy clusters, galaxies, and even stars.

How we Look at the Sound

Using the image of the CMB, we can measure the ripples of the sound waves using a Fourier analysis of the temperature gradients.

The temperature is used because it correlates to regions of high or low density, of course, the hotter a location is in space, the more matter managed to get in there, thus releasing more stuff.

A Fourier analysis is a very common technique in sound measurements. It takes any arbitrary mathematical function, and converts it into a frequency spectrum.

A Fourier analysis. On the top, the original signal, on the bottom, the signal's frequency spectrum Image Credit: The Mathematics of Waves and Materials Blog

We can conduct a Fourier analysis in two dimensions as well, we just perform two applications of the Fourier transform, but in two directions.

A 2D Fourier analysis. On the left, the original image, on the right, the image's 2D frequency spectrum Image Credit: Python Coding Block

We can then take this 2D image and plot it on a graph of the distance from the centre.

Applying the analysis and the plotting function to an image of the CMB we get this graph here:

Note that 'Multipole moment' is, for the sake of simplicity, a frequency spectrum Image Credit: Report of the Dark Energy Task Force, 2006

The above image is referred to as the power spectrum of the BAOs.

Adding constraints to Dark Matter/Energy

Dark Matter and Matter Density

The BAOs at the moment of them freezing become an incredibly useful measurement for the propagation of Dark Matter and Dark Energy.

Of course, dark matter is built into the equation, as they form a necessary component of the gravity force pulling things in. We can use the relative strength and period of the oscillations to estimate the amount of dark matter and it's ratio to Baryonic Matter.

Curvature

We can also use our graph for a second purpose: measuring the curvature of our universe.

You see, given the fact that our galaxies and galaxy clusters eventually end up forming from those regions of dense gas, and given that we know how 'far away' the CMB is (thanks to redshift).

We should know the approximate size of these dense gas regions in the CMB, given a certain curvature.

This will appear on our power spectrum as the shift in our graph, the location of our peaks along the x-axis.

In a universe that is hyperbolic, we should expect the density blobs to be small, and in a spherical universe, blobs that are larger. What we end up seeing is approximately a flat curvature.

Dark Energy

Now, thirdly, we can estimate the density of dark energy in our universe.

We can get this easily by measuring both matter density and curvature, as the two are related to the dark matter density.

But, we can also estimate it directly from analysing the power spectrum directly.

Dark energy affects the expansion of spacetime via the Friedmann equation, controlling for matter density, we can use the equation to quantify the Hubble parameter.

The size of the Hubble parameter, relative to our current Hubble constant, can be measured from the fact that the BAOs provide a standard measure of distance.

By analysing the power spectrum, we can effectively work backwards to derive a measurement for the Hubble parameter, and thus the presence of dark energy in the early universe.

Conclusion

Surprisingly a shorter-ish post this time, good on me.

Anyways I'll probably take a break for next week, as it's Christmas. Outside of that, I don't really know what else to say.

Follow if you want more, gib feedback on my post, and comment/repost your thoughts!

See ya later!

[gsunny6]

Y’all ever just vibe listening to some of that cmbr

[dat-physics-gal]

2.7 K isn't quite true. Like, yes, that is the spectrum of a blackbody radiator at 2.7 Kelvin, but it's not actually from today's space. It's a redshifted version of the time the universe went transparent. The past appears to us as though it is 2.7K hot, but most tiny dust particles in the milky way are about 16 to 20K hot. If it's closer to a star, it's even hotter.

And in intergalactic space... who knows. I'm sure someone also wrote a paper about it, but it's so far away it's probably much more error prone.

[planetariumhub]

The Majestic Eagle Nebula (M16): Unveiling the Stellar Sculptor

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) In the depths of the cosmos lies a breathtaking celestial masterpiece known as the Eagle Nebula, or Messier 16 (M16). Located in the constellation Serpens, this nebula has captivated astronomers and stargazers with its stunning beauty and unique features. In this article, we will embark on a journey to explore the enigmatic Eagle…