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We’ve known the Universe is expanding for a long time. The first solid paper demonstrating cosmic expansion was published by Edwin Hubble in 1929, based on observations made by Vesto Slipher, Milton Humason, and Henrietta Leavitt. Because of this, the rate of cosmic expansion is known as the Hubble constant, or Hubble parameter, H0. From this parameter, you can calculate things such as the age of the Universe since the Big Bang, so knowing the value of H0 is central to our understanding of modern cosmology. Early on, the measured value of the Hubble parameter varied widely. Hubble’s initial value was on the order of 500 (km/s)/Mpc. By the 1960s, the value settled down to between 50 and 90 (km/s)/Mpc, where it stayed for most of the 20th century. It was difficult to get more precise because our methods of calculating it were limited. All of these were based on the cosmic distance ladder, which uses a series of observations to calculate ever greater cosmic distances, each building on the previous method. But in the past few decades we got pretty good at it, and the Hubble value seemed to settle around 70 (km/s)/Mpc. After that, things started to get…problematic. With satellites such as WMAP and Planck we started to get high-resolution maps of the cosmic microwave background. From fluctuations in this background we have a new way to measure H0 and get a value of 67 – 68 (km/s)/Mpc. At the same time, observations of distant supernovae and the cosmic distance ladder pin down the value to 73 – 75 (km/s)/Mpc. Both methods are quite precise, and yet they entirely disagree. This disagreement is now known as the Hubble tension problem, and it is the most bothersome mystery in cosmology. Hubble tension between methods. Credit: Wikipedia user Primefac We aren’t sure what causes the Hubble tension. It might mean that one or more of our observation methods are fundamentally flawed, or it might mean there is something about dark energy and cosmic expansion that we really don’t understand. But astronomers generally agree that one way to address this mystery is to look for ways to measure H0 that are independent of both the cosmic background and the cosmic distance ladder. One such method involves gravitational lensing. Gravitational lensing occurs because gravity warps space, meaning that the path of light can be deflected by the presence of a large mass. So, for example, if a distant galaxy happens to be behind a closer galaxy from our vantage point, we see a gravitationally distorted view of the distant galaxy or even multiple images of the galaxy. The interesting thing about the multiple image effect is that the light from each image travels a different path around the closer galaxy, each with a different distance. Since the speed of light is finite this means each image gives us a view of the galaxy at different times in history. This doesn’t matter much for galaxies, but for supernovae it means gravitational lensing can let us observe the same supernova multiple times. By calculating the path of each supernova image we can determine the relative distance of each path, and by timing the appearance of each image we can determine the actual distance. This gives us a measurement that is independent of the cosmic distance ladder, giving us a new way to measure the Hubble parameter. This method has been used a couple of times, but the uncertainties of their Hubble values weren’t small enough to address the Hubble tension. However, a new study using this method is precise enough. The study is based on JWST images of a Type Ia supernova named SN H0pe. It is one of the most distant supernovae ever observed, and thanks to the less-distant galaxy cluster G165, the team captured three lensed images of SN H0pe. With their timing, observed brightness, and calculated paths, the team calculated H0 to be 70 – 83 (km/s)/Mpc. This still has a higher uncertainty than other methods, but it agrees with the usual distance ladder method. It also clearly disagrees with the cosmic microwave background method. Despite H0pe, the Hubble tension is very real. If anything, this new result makes the issue even more troublesome. There is something about cosmic expansion we don’t understand, and it’s now clear that better observations will not solve this mystery on their own. Reference: Pascale, Massimo, et al. “SN H0pe: The First Measurement of H0 from a Multiply-Imaged Type Ia Supernova, Discovered by JWST.” arXiv preprint arXiv:2403.18902 (2024). The post Gravitational Lens Confirms the Hubble Tension appeared first on Universe Today.

Could this be the next great comet? To be sure, these words have been said lots of times before. In a clockwork sky, how comets will perform is always the great wildcard. Comets from Kohoutek to ISON have failed to live up to expectations, while others like W3 Lovejoy took us all by surprise. But a discovery this past weekend has message boards abuzz, as an incoming sungrazer could put on a show right around Halloween. Anatomy of a Sungrazer The discovery comes to us from the prolific Asteroid Terrestrial-impact Last Alert System (ATLAS), which first spotted the comet on the night of September 27th. The initial designation of the comet was A11bP7I. The comet now has an official designation: C/2024 S1 ATLAS. This was announced on October 1st, in the International Astronomical Union’s Central Bureau for Astronomical Telegram’s message 5453. The orbit of Comet C/2024 S1 ATLAS. Credit: NASA/JPL. The highly eccentric hyperbolic orbit of the comet suggests it’s a member of the Kreutz family group of sungrazer comets. Most of these comets are doomed for destruction at perihelion, but there have been a few exceptions over the years. Those sungrazers that have survived have gone on to become great comets. Could C/2024 S1 ATLAS do the same? Comet Caveats Now, a few caveats are in order. Astronomers found S1 ATLAS at +12th magnitude, 1.094 Astronomical Units (AU) from the Sun. It could well be the case that it simply had an outburst right when it was first spotted, and could in fact be smaller and less energetic than it seems. What we need are more observations over the next few weeks. Comet C/2024 ATLAS imaged shortly after discovery. Credit: Michael Jaeger. “It’s early days, so I think the prudent approach is to moderate our expectations and then be ‘pleasantly surprised’ later,” astronomer Karl Battams (U.S. Naval Research Laboratory) told Universe Today. “That said, there’s clearly the potential for this to be a very exciting comet. The best analog we have is comet Lovejoy in 2011, which was discovered just a couple of weeks from perihelion, versus this one which is nearly a month away.” Comet S1 ATLAS imaged on September 28th. Credit: Filipp Romanov. The comet reaches perihelion on October 28th, 0.0082 AU from the Sun. That’s 762,600 miles from solar center, just 330,600 miles from the surface of the Sun. The solar radius is about 432,000 miles. As always seems to be the case, southern hemisphere observers will get a better view of the comet leading up to perihelion in mid-October as it approaches the Sun through the constellation Hydra. The comet will be visible low to the east at dawn, and ‘could’ break +6th magnitude in the final week of October. The comet passes 0.306 AU from the Earth on October 23rd after which, things could start to get interesting. Prospects for Sungrazer A1 ATLAS As of writing this, best estimates for peak magnitudes for comet S1 ATLAS top out at -7—think a bright daytime comet, but very close to the Sun—though -1st magnitude or so is probably more conservative. Northern hemisphere viewers might get best views of the comet low to the east at dawn after perihelion… if it survives. Looking low to the east at dawn on Halloween morning. Credit: Starry Night. “This Kreutz-group comet won’t pass quite as close to the Sun as W3 Lovejoy, so it’s not unreasonable to guess that it will aid its survival potential.” Says Battams. “Assuming so, it might be briefly visible to northern hemisphere observers very low in the early morning (in) southeast skies after perihelion, but it would require good viewing circumstances (a clear, low horizon)… and won’t hang around there for long.” A simulation of Comet A1 ATLAS in SOHO’s field of view. Credit: Starry Night. The comet enters the Solar Heliospheric Observatory (SOHO’s) LASCO C2/C3 field of view on October 26th, and exits on the 29th. It’s strange to think: prior to SOHO’s launch in 1995, astronomers knew of less than a handful of sungrazer comets. Now, thanks to the mission, we know of 5,065 sungrazing comets and counting. New sticky: I rarely tweet these days, mainly b/c most of the fun people have left. ? But I still pop in from time-to-time, and will post about exciting comet or Sun stuff.As always, any images/data I post are from 100% public sources, and all opinions are solely mine. pic.twitter.com/OeQRia2ppU — Karl Battams (@SungrazerComets) October 2, 2024 Classic Sungrazers of Yore 2011’s sungrazer W3 Lovejoy survived a passage just 87,000 miles from the surface of the Sun… Comet ISON, however, did not survive a 0.001244 AU, 116,000 mile surface pass at perihelion on U.S. Thanksgiving Day 2013. Long-time comet watchers will remember sungrazer Ikeya-Seki, which survived a 280,000 mile pass (just a little over the Earth-Moon distance) from the surface of the Sun. That comet went on to dazzle observers in 1965. Comet Ikeya-Seki. Credit: James W. Young/TMO/JPL/NASA. “What I will say is that I am very excited at the ‘prospect,’ and will be watching the evolution of this extremely closely over the next couple of weeks.” says Battams. “I think by mid-October we’ll be able to state some facts with a lot more certainty.” It seems like good comets always come in pairs…remember Hale-Bopp and Hyakutake in the late 90s? We (finally) caught sight of comet C/2023 A3 Tsuchinshan-ATLAS this morning from here in Bristol, Tennessee, looking like a fuzzy ‘star’ with a short tail in the brightening twilight low to the east, peeking out between pine trees. We’re cautious for now when it comes to S1 ATLAS. But remember: comets never read predictions… and S1 ATLAS could well surprise us. The post Could a New Sungrazer Comet Put on a Show at the End of October? appeared first on Universe Today.

Blurring the line between internal and external realities.

She memorably covered "I Want to Know What Love Is" several years ago. Continue reading…