What are these bubbles? What is Dark Matter and Dark Energy?
The Fermi bubbles you see above were discovered by a team of scientists working at the Harvard-Smithsonian Centre for Astrophysics, using the Fermi Gamma-ray Space Telescope (FGST).
It shows two giant gamma and X- ray blob-like clouds that stretch for thousand of light years above and below the galactic centre.
Back in the 1990’s, clues of the bubbles lower outer edge (blue in the image) were first observed in X-rays using the X-ray telescope Röntgensatellit (ROSAT).
The FGST further allowed researchers to see the full extent of the bubble by mapping the gamma radiation (magenta), and so we have this stunning conceptual image.
According to this Australian- US led study, an enormous, violent explosion erupted from the centre of the Milky Way around 3.5 million years ago, which is likely to be the location of a supermassive black hole named Sagittarius A*.
Scientists now believe that these highly radiated Fermi bubbles are the remnants of that explosion which may have lasted up to 300,000 years.
And given the explosion occurred relatively recently in galactic terms, this means that a segment of our early human ancestry would have witnessed this extraordinary event.
Lead author of the study Professor Joss Bland-Hawthorn said this;
“The flare must have been a bit like a lighthouse beam. Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time.”
This has changed our understanding of our galaxy ultimately, since the Milky Way was previously thought to be an inactive galaxy, with a not so active black hole at its centre.
The discovery of such Fermi bubbles is now challenging this initial observation.
It’s still a mystery however as to what precisely caused the event to happen in the first place, with some astronomers referring to Seyfert flares.
Around 10% of all galaxies are classified as Seyfert Galaxies, which are part of the largest groups of active galaxies in the Universe.
These galaxies have supermassive black holes at their cores, surrounded by accretion discs of in-falling matter.
Occasionally, something particularly massive may fall victim to the black hole, causing it to spew out highly energised jets. Perhaps then Sag A* went through some similar event. Until further research is carried out however, the explosion remains unexplained.
Dark Matter and Dark Energy
Dark matter and dark energy are said to make up 95% of the total mass-energy of the Universe.
This means that normal everyday matter in which you and I made of; including every star and galaxy in the Universe, only makes up 5%.
This is a sobering thought considering the rate of scientific and technological progress made in the last couple centuries, yet we’ve only just discovered we’re unable to account for the majority of the environment which our planet, life, and all matter resides.
As the name implies, dark matter is such because we’ve yet to directly observe it with our scientific instruments.
There is no seeming interaction between observable electromagnetic radiation such as light, and so is invisible to the entire electromagnetic spectrum.
Its interaction with ordinary, baryonic matter, is almost non-existent, except through gravity. This essentially makes it impossible to observe directly with current technology.
The idea that the majority of the energy-mass of the Universe is unaccountable matter was established in the 1930’s.
Swiss Astronomer Fritz Zwicky showed upon observation of the Coma Cluster (a cluster of a thousand galaxies) that each galaxy was many times more massive than inferred by their stellar constituents. He used the virial theorem to infer the existence of unseen matter, which he referred to as dunkle Materie — dark matter.
Then in the 1970’s, pioneering work done by astronomer Vera Rubin mounted further evidence for the case for dark matter.
She discovered that stars orbiting the outer edge of a spiral galaxies disc travel as fast as those close to the centre where most of the galactic mass resides. Uncovering the discrepancy between the predicted angular motion, and the observed motion of galaxies, she showed that spiral galaxies should fly apart if all that was keeping them together was the sum total mass of their stars.
However since they don’t, a large undetectable mass must be holding them together — an early indication of dark matter halos.
We can now infer the existence of dark matter through the observation of gravitational lensing; a predicted consequence of Einsteins general relativity.
Since mass causes the bending of space-time via gravity, it will also bend any form of electromagnetic radiation, such as visible light or radio waves in some particular way.
Astronomers look for regions in space that are particularly massive such as galaxy clusters, that sit in between Earth and some other distance bright object such as another galaxy or quasar.
By measuring how radiation from these distant objects are distorted by the cluster, we can estimate how massive the cluster is. Then by comparing this to the amount of visible matter, we’re able to get a rough idea of the distribution of dark matter in the cluster.
We Know it Exists. So what it is?
Many scientists believe that dark matter is non-baryonic in nature, therefore inferring some yet undiscovered sub-atomic particle or particles.
Indeed most physicists agree that the standard model of particle physics falls short of a unified theory of the fundamental interactions between particles. A full explanation of dark matter then is likely part of the missing picture, along with dark energy.
There are a number of proposed explanations for dark matter that include, but are not limited to: weakly interacting massive particles (WIMP’s), sterile neutrinos, supersymmetry, extra dimensions, super fluid vacuum theory, and also modified theories of gravity such entropic gravity.
Of course some theories hold more weight than others. WIMP’s were once considered one of the more promising theories, however, professor of Physics John Terning, at UC Davis in a phys.org article notes that;
“The primary candidate for a long time was the WIMP, but it looks like that’s almost completely ruled out.”
That being said, in 2020, the LUX-Zeplin (LZ) experiment is set to start collecting data to try and detect whether these WIMP’s exist.
Briefly, the experiment works by triggering a unique sequence of light and electrical signals in a huge tank filled with up to 10 tons of purified liquid xenon.
Xenon is used due to its unique properties, allowing them to produce light in specific particle interactions.
It’s thought that as dark matter collides with these substances, faint shimmers of light would be emitted allowing us directly observe its properties.
Likewise, scientists in China are set to complete the Particle and Astrophysical Xenon Detector (PandaX) which will observe up to 4 tons of xenon, as well as the XENON experiment in Italy due for an upgrade to facilitate 8 tons of xenon.
And finally, the Laser Interferometer Gravitational-Wave observatory (LIGO), responsible for confirming Einsteins gravitational waves, continued gathering data this year, and may very well shine more light on the mysteries of dark matter.
The Large Synoptic Survey Telescope (LSST) currently under construction, is expected to capture first light next year.
It’s hoped that this experiment may help understand the large scale structure of dark matter throughout the Universe, its interaction with baryonic matter, and large visible structures such as galaxies and galaxy clusters.
Dark Energy is a much more recently discovered phenomena, and is seemingly far more mysterious than dark matter.
Einstein famously introduced the cosmological constant into is theory of general relativity in 1917, in order to account for the attractive force of gravity in an assumed static Universe.
However in 1929, Edwin Hubble showed that the Universe is in fact expanding, by observing the light of distant galaxies being shifted towards the red end of the light spectrum.
This showed that distant galaxies are moving away from us. Hubble then compared the estimated distance of galaxies with their red shift and came up with Hubble’s law.
In 1998, two international teams of astronomers further inferred the existence of a mysterious force, dark energy, by showing that the rate of the Universe’s expansion is in fact speeding up — accelerating expansion.
This was achieved by observing the apparent brightness of objects of known luminosity like Type Ia supernovas.
These types of supernovas that exploded when the Universe was smaller in size were less bright, therefore further away than they would be if the Universe had no dark energy. Suggesting that the expansion rate is indeed faster now than was it was.
The precise nature of dark energy remains unknown however.
It’s thought to be homogeneous throughout the Universe, accounting for up to 68% of the energy density, and therefore its mass.
And yet given this, dark energy does not interact with any of the fundamental forces of nature besides gravity in some intrinsic way.
The energy density of dark energy is inherent to what we observe to be the vacuum of space.
Presently, scientists believe that accelerated expansion is caused by some repulsive force generated by quantum fluctuations permeating all of space, and seems to be getting stronger.
Many scientists have proposed that dark energy could be a fifth fundamental force of nature, along with, gravity, electromagnetism and the strong and weak nuclear forces.
As is the case with dark matter, much observation and research is needed if we’re to understand the true nature of our Universe.
There is a great deal of discovery to be made.
Thanks for reading and sharing!