- 3363 - GALAXIES - beyond were light can see? The present “reachability limit” has a boundary 18 billion light-years away. The limit of the visible universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years.
--------------------- 3363 - GALAXIES - beyond were light can see?
- Our universe, everywhere and in all directions, is filled with stars and galaxies. Beyond our Milky Way galaxy are trillions of others galaxies, nearly all of which are expanding away from us.
-
- From our vantage point, we observe up to 46,100,000,000 light-years away. As long as the light from any galaxy that was emitted at the start of the hot Big Bang 13.8 billion years ago would have reached us by today, that object is within our presently observable universe. However, not every observable object is reachable.
-
- Our visible universe contains an estimated 2 trillion galaxies. The “Hubble eXtreme Deep Field” (XDF) survey may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover 5,500 galaxies within it. This is an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice.
-
- The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal. Most of them are already permanently unreachable by us.
-
- Although there are magnified, ultra-distant, very red and even infrared galaxies in the extreme Deep Field, there are galaxies that are even more distant out there than what we’ve discovered in our deepest-to-date views. These galaxies will always remain visible to us, but we will never see them as they are today, 13.8 billion years after the Big Bang.
-
- As the universe expands, the space between all unbound objects increases over time.
Light redshifts and distances between unbound objects change over time in the expanding universe. The objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them.
-
- Beyond distances of 14.5 billion light-years, space’s expansion pushes galaxies away faster than light can travel.
-
- Over time, the expansion rate still drops, but remains positive and large because of “dark energy“. The expected fates of the universe all correspond to a universe where the matter and energy combined fight against the initial expansion rate. In our observed universe, a cosmic acceleration is caused by some type of unexplained dark energy.
-
- All of these universes are governed by the Friedmann equations, which relate the expansion of the universe to the various types of matter and energy present within it.
Dark energy, inherent to space itself, never decreases, even as the universe expands.
-
- Radiation , and a cosmological constant all evolve with time in an expanding universe. As the universe expands, the “matter density dilutes“, but the “radiation” also becomes cooler as its wavelengths get stretched to longer, less energetic states. Dark energy’s density will truly remain constant as a form of energy intrinsic to space itself.
-
- All galaxies beyond a certain distance always remain unreachable, even at the speed of light. Our deepest galaxy surveys can reveal objects tens of billions of light years away, but there are more galaxies within the observable universe we still have yet to reveal.
-
- There are parts of the universe that are not yet visible today that will someday become observable to us, and there are parts that are visible to us that are no longer reachable by us, even if we traveled at the speed of light.
-
- The present “reachability limit” has a boundary 18 billion light-years away. The limit of the visible universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years.
-
- However, beyond about 18 billion light-years, we can never access a galaxy even if we traveled towards it at the speed of light. All galaxies closer than that could be reached if we left today; all galaxies beyond that are unreachable.
-
- Given enough time, light that was emitted by a distant object will arrive at our eyes, even in an expanding universe. However, if a distant galaxy’s recession speed reaches and remains above the speed of light, we can never reach it, even if we can receive light from its distant past.
- Only 6% of presently observable galaxies remain reachable; 94% already lie beyond our reach.
-
- The “GOODS-North survey“, contains some of the most distant galaxies ever observed, a great many of which are already unreachable by us. As time marches forward, more and more galaxies suffer this same fate, disconnecting from us. Each year, another 160 billion stars, enough to compose one major galaxy, become newly unreachable.
-
- The stars, in the M81 group, will become unreachable after another 100 billion years. Located a mere 3.6 Megaparsecs away from our Local Group, the M81 group is the nearest substantial group of galaxies to our own Local Group, but will remain gravitationally unbound.
-
- After 100 billion years, even these galaxies will become unreachable to us, even if we were to leave at the speed of light. After that, only our “Local Group of Galaxies” will remain within reach.
-
- The Local Group of galaxies is dominated by Andromeda and the Milky Way, and additionally consists of about 60 other, smaller galaxies. All are located within 5 million light-years of one another, with the nearest galactic groups beyond our own remaining gravitationally unbound from ourselves for all-time.
-
- Our understanding of blackholes is now central to our understanding of the cosmos. The next generation “Very Large Array” (ngVLA) will help astronomers study these mysterious objects.
-
- Astronomers have long known that Einstein's theory of gravity allowed for an object to be so massive that light itself could not escape, but they initially doubted that blackholes existed in the Universe.
-
- Today blackholes are recognized as a standard result of the death of very massive stars.
A century ago, astronomers thought that the Universe consisted mostly of stars. They shine with the colors of light that our human eyes can see, and to most of us, the picture of an astronomer includes a telescope turned to the heavens.
-
- Today, we now recognize that a variety of objects shine at wavelengths that our eyes cannot see, from long wavelength radio waves to extremely high-energy gamma rays. We now know that there are a variety of other messengers carrying to us information about the Universe.
-
- Cosmic rays are energetic sub-atomic particles, with energies well above those that particle accelerators such as the Large Hadron Collider can produce. In the most extreme cases, a sub-atomic particle can hit the Earth's atmosphere with as much energy as a fast-pitch baseball.
-
- Billions of neutrinos rain upon us every second. They are born from nuclear fusion in the Sun, from distant exploding stars, and from the regions near supermassive blackholes. And gravitational waves constantly wash over the Earth and the Solar System. These distortions of spacetime itself are generated by colliding blackholes, and potentially by the expansion of the Universe.
-
- Within our own Milky Way, the “ngVLA” will greatly expand our ability to detect blackholes in binary systems, enabling probes of supernova explosions and blackhole formation. It will also enable the detection of less massive blackholes that dwell in the centers of dwarf galaxies throughout the local cluster.
-
- The detection of gravitational waves, along with their direct link to merging compact objects, marks one of the major breakthroughs in astrophysics over the past 10 years. The ngVLA will be able to resolve and observe the motion of mergers of supermassive black holes and neutron stars, both sources of gravitational waves.
-
- New facilities can detect these merging stellar remnants in galaxies up to 600 million light-years away through the gravitational wave and neutrino events they produce, and the ngVLA will be able to detect the radio emission to the same distance, permitting us to determine the physical conditions at the location of neutrino production.
-
- Even the most supermassive of the supermassive blackholes aren’t very large, making it extremely difficult to measure their sizes. However, astronomers have recently developed a new technique that can estimate the mass of a blackhole based on the movement of hot gas around them even when the blackhole itself it smaller than a single pixel.
-
- Supermassive blackholes are surrounded by tons of superheated plasma. That plasma swirls around the backhole, forming a torus and an accretion disk that continually feeds material into the blackhole. Because of the extreme gravity, that gas moves incredibly quickly and shines fiercely. It’s that light that we identify as a “quasar‘, which can be seen from across the universe.
-
- While the quasars are relatively easy to spot, it’s much more challenging to quantify the properties of the central blackhole. Now, for the first time, in 2021, Astronomers are demonstrating the feasibility of directly determining the mass of a quasar using a technique called spectroastrometry.
-
- “Spectroastrometry’ relies on observing the area around the blackhole. As the gas swirls around it, some of it will be moving in our direction and some if it will be moving away. The portion of the gas moving towards us will be blue-shifted, and the portion moving away will shift more red.
-
- Even if the central blackhole and accretion disk are too small to resolve, this technique can still be applied to regions further away, and through modeling the researchers can estimate a mass.
-
- By separating spectral and spatial information in the collected light, as well as by statistically modeling the measured data, astronomers can derive distances of much less than one image pixel from the center of the accretion disk.
-
- This technique was applied to “J2123-0050“, a quasar active when the universe was just 2.9 billion years old. They found that the central black hole weighed 1.8 billion solar masses.
-
- With the significantly increased sensitivity of the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT), with a primary mirror diameter of 39 meters, currently under construction, we will soon be able to determine quasar masses at the highest redshifts
-
November 29, 2021 GALAXIES - beyond were light can see? 3363
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Tuesday, November 30, 2021 ---------------------------
No comments:
Post a Comment