5 ms·

"It will be huge, but the gravitational waves LIGO would detect are not primordial or from inflation, but rather from supernovae and compact binary mergers (neu

"It will be huge, but the gravitational waves LIGO would detect are not primordial or from inflation, but rather from supernovae and compact binary mergers (neutron star pairs, stellar-mass black hole pairs, etc.)." I missed this. How do they tell the difference? (Interferomety?)

privong 11y ago
Generally, the difference is direct vs indirect detection. LIGO aims to directly detect gravitational waves as they pass Earth by measuring the change in length of two parallel arms (using interferometry). In contrast, BICEP2 was looking for a polarization signal in the cosmic microwave background (CMB) which would occur if there were primordial gravitational waves (but this signal could also be mimicked by foreground dust). So BICEP2's claimed detection wasn't a direct detection of gravitational waves. BICEP2 detected polarization (specifically B-mode polarization[0]) in the CMB. The BICEP2 team initially thought the amount of foreground dust was too low to explain the degree of polarization they observed, so they attributed the polarization to primordial gravitational waves affecting the cosmic microwave background. As you noted, Planck had multiple bands and so could more effectively determine the amount of foreground dust. Using a joint BICEP2+Planck dataset, the teams determined the signal could be explained by foreground dust. In contrast, LIGO is attempting to directly measure gravitational waves from: 1) discrete, individual events, such as a merger of neutron stars. As the objects inspiral, they emit gravitational waves, with the emission becoming stronger (and, presumably higher frequency) as the objects near final merger. At final coalescence, a burst of gravitational waves is emitted. It is hoped/expected that this burst (plus some GW emission leading up to the merger) will be detected by LIGO for individual sources. 2) A gravitational wave background from the ensemble of events too far away to detect individually, but whose sum adds up to enough to be detectable. In contrast to the above, this will be some linear combination of all the passing waves from distant individual events, from a variety of directions. The frequency of the gravitational waves depends on the bodies involved. With LIGO's 2km arms, it is sensitive to a range of frequencies which correspond to expectations for stellar-mass compact binaries. The LISA project[1] would have longer baselines and so would be sensitive to mergers of more massive objects, such as supermassive black hole pairs. So, the baseline of your gravitational wave experiment determines what you can possibly measure. This graphic shows at which frequencies various phenomena emit[2]. Based on that graphic, LIGO may be sensitive to the frequencies corresponding to primordial gravitational waves, but they are likely far far below LIGO's detection ability. My impression is that direct detection of primordial gravitational waves will not be possible for a while, if ever. Rather, they may be detected indirectly, via the imprint they leave on the CMB polarization (as BICEP2 tried). [0] https://en.wikipedia.org/wiki/Cosmic_microwave_background#Polarization https://en.wikipedia.org/wiki/Cosmic_microwave_background#Po... [1] http://lisa.nasa.gov/ http://lisa.nasa.gov/ [2] https://en.wikipedia.org/wiki/File:The_Gravitational_wave_spectrum_Sources_and_Detectors.jpg https://en.wikipedia.org/wiki/File:The_Gravitational_wave_sp... EDIT: Added new first paragraph and reworded the rest for clarity/flow.
- bootload 11y ago
  thanks @privong, this is a first rate explanation of LIGO/BICEP2 and CMB and a major reason I hang out on HN.
  - privong 11y ago
    Sure thing, @bootload. By the way, I should have mentioned that we already have indirect evidence for gravitational waves, from the orbital decay of binary pulsars such as PSR B1913+16 [0]. The observed orbital decay has an excellent match with the orbital decay predicted using general relativity and gravitational waves. [1] shows the agreement, with the line being a theoretical prediction and not a fit. [0] https://en.wikipedia.org/wiki/PSR_B1913%2B16 https://en.wikipedia.org/wiki/PSR_B1913%2B16 [1] https://en.wikipedia.org/wiki/PSR_B1913%2B16#/media/File:PSR_B1913%2B16_period_shift_graph.svg https://en.wikipedia.org/wiki/PSR_B1913%2B16#/media/File:PSR...

privong 11y ago

Generally, the difference is direct vs indirect detection. LIGO aims to directly detect gravitational waves as they pass Earth by measuring the change in length of two parallel arms (using interferometry). In contrast, BICEP2 was looking for a polarization signal in the cosmic microwave background (CMB) which would occur if there were primordial gravitational waves (but this signal could also be mimicked by foreground dust). So BICEP2's claimed detection wasn't a direct detection of gravitational waves. BICEP2 detected polarization (specifically B-mode polarization[0]) in the CMB. The BICEP2 team initially thought the amount of foreground dust was too low to explain the degree of polarization they observed, so they attributed the polarization to primordial gravitational waves affecting the cosmic microwave background. As you noted, Planck had multiple bands and so could more effectively determine the amount of foreground dust. Using a joint BICEP2+Planck dataset, the teams determined the signal could be explained by foreground dust. In contrast, LIGO is attempting to directly measure gravitational waves from: 1) discrete, individual events, such as a merger of neutron stars. As the objects inspiral, they emit gravitational waves, with the emission becoming stronger (and, presumably higher frequency) as the objects near final merger. At final coalescence, a burst of gravitational waves is emitted. It is hoped/expected that this burst (plus some GW emission leading up to the merger) will be detected by LIGO for individual sources. 2) A gravitational wave background from the ensemble of events too far away to detect individually, but whose sum adds up to enough to be detectable. In contrast to the above, this will be some linear combination of all the passing waves from distant individual events, from a variety of directions. The frequency of the gravitational waves depends on the bodies involved. With LIGO's 2km arms, it is sensitive to a range of frequencies which correspond to expectations for stellar-mass compact binaries. The LISA project[1] would have longer baselines and so would be sensitive to mergers of more massive objects, such as supermassive black hole pairs. So, the baseline of your gravitational wave experiment determines what you can possibly measure. This graphic shows at which frequencies various phenomena emit[2]. Based on that graphic, LIGO may be sensitive to the frequencies corresponding to primordial gravitational waves, but they are likely far far below LIGO's detection ability. My impression is that direct detection of primordial gravitational waves will not be possible for a while, if ever. Rather, they may be detected indirectly, via the imprint they leave on the CMB polarization (as BICEP2 tried). [0] https://en.wikipedia.org/wiki/Cosmic_microwave_background#Polarization https://en.wikipedia.org/wiki/Cosmic_microwave_background#Po... [1] http://lisa.nasa.gov/ http://lisa.nasa.gov/ [2] https://en.wikipedia.org/wiki/File:The_Gravitational_wave_spectrum_Sources_and_Detectors.jpg https://en.wikipedia.org/wiki/File:The_Gravitational_wave_sp... EDIT: Added new first paragraph and reworded the rest for clarity/flow.

bootload 11y ago
thanks @privong, this is a first rate explanation of LIGO/BICEP2 and CMB and a major reason I hang out on HN.
- privong 11y ago
  Sure thing, @bootload. By the way, I should have mentioned that we already have indirect evidence for gravitational waves, from the orbital decay of binary pulsars such as PSR B1913+16 [0]. The observed orbital decay has an excellent match with the orbital decay predicted using general relativity and gravitational waves. [1] shows the agreement, with the line being a theoretical prediction and not a fit. [0] https://en.wikipedia.org/wiki/PSR_B1913%2B16 https://en.wikipedia.org/wiki/PSR_B1913%2B16 [1] https://en.wikipedia.org/wiki/PSR_B1913%2B16#/media/File:PSR_B1913%2B16_period_shift_graph.svg https://en.wikipedia.org/wiki/PSR_B1913%2B16#/media/File:PSR...