The Event Horizon Telescope (EHT) has recently mapped the central compact object of the galaxy M87 with an unprecedented angular resolution. Though the remarkable breakthrough has been interpreted based on theory that M87 contains a rotating or "Kerr" black hole.
New research published in EPJ C by Chandrachur Chakraborty and Qingjuan Yu at the Kavli Institute for Astronomy and Astrophysics, Peking University (KIAA-PKU), Masoumeh Ghasemi-Nodehi and Youjun Lu, at the National Astronomical Observatories of China, looks at possible alternative explanations for the image.
"The EHT collaboration has tried to show that the observed image is overall consistent with the expectations for the shadow of a Kerr black hole," Chakraborty says. "As the alternatives to the Kerr BH have not been ruled out, we have investigated whether the EHT data is also consistent or not with alternative models for the central object of M87."
Chakraborty goes on to explain that he and his co-authors had one primary purpose to show how a gravitomagnetic monopole?-?or an NUT parameter?-?affects the shadow size and shape, and whether its existence can be ruled out or not in M87*.
"To show this, we use the observational parameter values of the first image of M87* and found that a non-zero gravitomagnetic monopole is still compatible with the current EHT observations," Chakraborty says.
Chakraborty goes on to explain what a gravitomagnetic monopole is: "In nature, north and south magnetic poles always go hand in hand. Cutting a bar magnet in half just creates two magnets, each of which still has two poles, rather than creating separate north and south poles on each half. Yet their electrostatic cousins, positive and negative charges, exist independently."
The researcher adds that in theoretical physics, gravity and electromagnetism have analogous features. "Mass is considered as the analogous to electric charge. Therefore, we call mass the gravitoelectric charge," Chakraborty says. "The next question is does gravitomagnetic charge or the so-called gravitomagnetic monopole exist in nature?"
In the paper the authors propose that M87* may contain a gravitomagnetic monopole, and, therefore, could be described as a more general Kerr-Taub-NUT spacetime, with Kerr spacetime a special case of the Kerr-Taub-NUT spacetime with vanishing gravitomagnetic monopole.
"In that sense, no models are incorrect, and this basically put a strong constraint on the spacetime structure of the central compact radio source in M87." Chakraborty concludes, adding how competing theories could be tested.
"Essentially, accurate measurements of both the shadow size and asymmetry could put strong constraints on Kerr parameter and NUT parameter, and break the degeneracies between the Kerr and Kerr-Taub-NUT spacetimes, including those between the black holes and naked singularities."
–SPACE STORY-
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ED https://www.eurekalert.org/news-releases/934899
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22-DEC-49
Liquid fuels from carbon dioxide
Liquid fuels from carbon dioxide
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by Staff Writers
Washington DC (SPX) Nov 16, 2021
A new electrocatalyst called a-CuTi@Cu converts carbon dioxide (CO2 ) into liquid fuels. As reported by a team of Chinese researchers in the journal Angewandte Chemie, active copper centered on an amorphous copper/titanium alloy produces ethanol, acetone, and n-butanol with high efficiency.
Most of our global energy demands are still being met by burning fossil fuels, which contributes to the greenhouse effect through the release of CO2 . To reduce global warming, we must look for opportunities to use CO2 as a raw material for basic chemicals. Through electrocatalytic conversion of CO2 using renewable energy, a climate-neutral, artificial carbon cycle could be established.
Excess energy produced by photovoltaics and wind energy could be stored through the electrocatalytic production of fuels from CO2. These could then be burned as needed. Conversion into liquid fuels would be advantageous because they have high energy density and are safe to store and transport. However, the electrocatalytic formation of products with two or more carbon atoms (C2+) is very challenging.
A team from Foshan University (Foshan, Guangdong), the University of Science and Technology of China (Hefei, Anhui), and Xi'an Shiyou University (Xi'an, Shaanxi), led by Fei Hu, Tingting Kong, Jun Jiang, and Yujie Xiong has now developed a novel electrocatalyst that efficiently converts CO2 to liquid fuels with multiple carbon atoms (C2-4). The primary products are ethanol, acetone, and n-butanol.
To make the electrocatalyst, thin ribbons of a copper/titanium alloy are etched with hydrofluoric acid to remove the titanium from the surface. This results in a material named a-CuTi@Cu, with a porous copper surface on an amorphous CuTi alloy.
It has catalytically active copper centers with remarkably high activity, selectivity, and stability for the reduction of CO2 to C2+ products (total faradaic efficiency of about 49 % at 0.8 V vs. reversible hydrogen electrode for C2-4, and it is stable for at least three months). In contrast, pure copper foil produces C1 products but hardly any C2+ products.
The reaction involves a multistep electron-transfer process via various intermediates. In the new electrocatalyst, the inactive titanium atoms below the surface actually play an important role; they increase the electron density of the Cu atoms on the surface.
This stabilizes the adsorption of *CO, the key intermediate in the formation of multicarbon products, allows for high coverage of the surface with *CO, and lowers the energy barrier for di- and trimerization of the *CO as new carbon-carbon bonds are formed.
