K. Griest

University of California, San Diego, San Diego, California, United States

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Publications (230)713.61 Total impact

  • Agnieszka Cieplak · K. Griest · M. Lehner
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    ABSTRACT: Primordial Black Holes (PBHs) remain one of the few Dark Matter (DM) candidates left within the Standard Model of Particle Physics. We have previously found that previous PBH DM limits could theoretically be extended by two orders of magnitude by using the microlensing of the source stars monitored by the Kepler satellite due to its photometric precision and the large projected cross section of the nearby stars. Here we present the experimental results of our study of the first two years of Kepler stellar lightcurves. After eliminating background events such as variable stars, flares, and comets, we have found no microlensing events. We were therefore able to calculate our efficiency of detection by introducing millions of fake microlensing events which included limb-darkening and a corrected finite-source microlensing formalism. By performing this Monte Carlo analysis, we have found that PBHs with masses between 2 × 10-9 M⊙ and 10-7 M⊙ cannot constitute the entirety of the DM, thereby constraining a full order of magnitude of the previously allowed PBH DM mass range.
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    ABSTRACT: We present new limits on the allowed masses of a dark matter (DM) halo consisting of primordial black holes (PBH) (or any other massive compact halo object). We analyze two years of data from the Kepler satellite, searching for short-duration bumps caused by gravitational microlensing. After removing background events consisting of variable stars, flare events, and comets or asteroids moving through the Kepler field, we find no microlensing candidates. We measure the efficiency of our selection criteria by adding millions of simulated microlensing lensing events into the Kepler light curves. We find that PBH DM with masses in the range 2×10^{-9}M_{⊙} to 10^{-7}M_{⊙} cannot make up the entirety of the DM in the Milky Way. At the low-mass end, this decreases the allowed mass range by more than an order of magnititude.
    Physical Review Letters 11/2013; 111(18):181302. DOI:10.1103/PhysRevLett.111.181302 · 7.51 Impact Factor
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    Joseph M. Fedrow · Kim Griest
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    ABSTRACT: A cosmological constant fits all current dark energy data, but requires two extreme fine tunings, both of which are currently explained by anthropic arguments. Here we discuss anti-anthropic solutions to one of these problems: the cosmic coincidence problem- that today the dark energy density is nearly equal to the matter density. We replace the ensemble of Universes used in the anthropic solution with an ensemble of tracking scalar fields that do not require fine-tuning. This not only does away with the coincidence problem, but also allows for a Universe that has a very different future than the one currently predicted by a cosmological constant. These models also allow for transient periods of significant scalar field energy (SSFE) over the history of the Universe that can give very different observational signatures as compared with a cosmological constant, and so can be confirmed or disproved in current and upcoming experiments.
    Journal of Cosmology and Astroparticle Physics 09/2013; DOI:10.1088/1475-7516/2014/01/004 · 5.81 Impact Factor
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    ABSTRACT: We present the analysis on our new limits of the dark matter (DM) halo consisting of primordial black holes (PBHs) or massive compact halo objects (MACHOs). We present a search of the first two years of publicly available Kepler mission data for potential signatures of gravitational microlensing caused by these objects, as well as an extensive analysis of the astrophysical sources of background error. These include variable stars, flare events, and comets or asteroids which are moving through the Kepler field. We discuss the potential of detecting comets using the Kepler lightcurves, presenting measurements of two known comets and one unidentified object, most likely an asteroid or comet. After removing the background events with statistical cuts, we find no microlensing candidates. We therefore present our Monte Carlo efficiency calculation in order to constrain the PBH DM with masses in the range of 2 x 10^-9 solar masses to 10^-7 solar masses. We find that PBHs in this mass range cannot make up the entirety of the DM, thus closing a full order of magnitude in the allowed mass range for PBH DM.
    The Astrophysical Journal 07/2013; 786(2). DOI:10.1088/0004-637X/786/2/158 · 5.99 Impact Factor
  • Agnieszka Cieplak · K. Griest
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    ABSTRACT: Primordial Black Holes (PBHs) remain a viable Dark Matter (DM) candidate of the Standard Model of Particle Physics. Previously, we have proposed a new method to constrain the remaining PBH DM mass range using microlensing of Kepler source stars, with the possibility of closing up to 40% of the remaining mass window. Here we re-address this analysis using a more accurate treatment of the distribution of the source stars, including limb-darkening as well as reflecting a more accurate number of variable stars. Including the extended Kepler mission the theoretically detectable PBH DM mass range could be extended down to 2*10^-10 solar masses. We address the possible PBH parameters that could be detected if such an event would be observed as well as possible improvements for future survey satellite missions.
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    Agnieszka M. Cieplak · Kim Griest
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    ABSTRACT: Primordial Black Holes (PBHs) remain a Dark Matter (DM) candidate of the Standard Model of Particle Physics. Previously, we proposed a new method of constraining the remaining PBH DM mass range using microlensing of stars monitored by NASA's Kepler mission. We improve this analysis using a more accurate treatment of the population of the Kepler source stars, their variability and limb-darkening. We extend the theoretically detectable PBH DM mass range down to $2\times10^{-10} M_\sun$, two orders of magnitude below current limits and one third order of magnitude below our previous estimate. We address how to extract the DM properties such as mass and spatial distribution if PBH microlensing events were detected. We correct an error in a well-known finite-source limb-darkening microlensing formula and also examine the effects of varying the light curve cadence on PBH DM detectability. We also introduce an approximation for estimating the predicted rate of detection per star as a function of the star's properties, thus allowing for selection of source stars in future missions, and extend our analysis to planned surveys, such as WFIRST.
    The Astrophysical Journal 10/2012; 767(2). DOI:10.1088/0004-637X/767/2/145 · 5.99 Impact Factor
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    ABSTRACT: Primordial Black Holes (PBHs) are the only remaining Dark Matter (DM) candidate of the Standard Model of Particle Physics. We present a new method of constraining up to 40% of the remaining mass range of the PBH DM using microlensing of stars targeted by NASA's Kepler mission. Kepler's exceptional photometric precision and finite-source effects allow for a higher microlensing rate than previously thought. We introduce a new formalism with these effects for the optical depth and microlensing rate.
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    ABSTRACT: If the dark matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extrasolar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 light curves would result in large numbers of detectable events for PBHs in the mass range 5×10(-10) M(⊙) to 10(-4) M(⊙). Nondetection of these events would close almost 2 orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.
    Physical Review Letters 12/2011; 107(23):231101. DOI:10.1103/PhysRevLett.107.231101 · 7.51 Impact Factor
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    ABSTRACT: We attempt to measure possible miscalibration of the wavelength scale of the VLT-UVES spectrograph. We take spectra of QSO HE0515-4414 through the UVES iodine cell which contains thousands of well-calibrated iodine lines and compare these lines to the wavelength scale from the standard thorium-argon pipeline calibration. Analyzing three exposures of this z = 1.71 QSO, we find two distinct types of calibration shifts needed to correct the Th/Ar wavelength scale. First, there is an overall average velocity shift of between 100 m/s and 500 m/s depending upon the exposure. Second, within a given exposure, we find intra-order velocity distortions of 100 m/s up to more than 200 m/s. These calibration errors are similar to, but smaller than, those found earlier in the Keck HIRES spectrometer. We discuss the possible origins of these two types of miscalibration. We also explore the implications of these calibration errors on the systematic error in measurements of the relative change in alpha (current value - past value) / current value, the change in the fine-structure constant derived from accurate measurement of the relative redshifts of absorption lines in QSO absorption systems. The overall average, exposure-dependent shifts should be less relevant for fine-structure work, but the intra-order shifts have the potential to affect these results. Using either our measured calibration offsets or a Gaussian model with sigma of around 90 m/s, Monte Carlo mock experiments find errors in the relative change in alpha of between 1e-6 Nsys^(-1/2) and 3e-6 Nsys^(-1/2), where Nsys is the number of systems used and the range is due to dependence on how many metallic absorption lines in each system are compared. Comment: 12 pages, 8 figures, accepted ApJ
    The Astrophysical Journal 04/2010; 723(1). DOI:10.1088/0004-637X/723/1/89 · 5.99 Impact Factor
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    ABSTRACT: We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of determining whether the fine structure constant was different in the past. Using new software and techniques, we measured the redshifts of various Ni II, Fe II, Si II, etc. lines in a damped Ly-alpha system at z=2.309. Roughly half the data was taken through the Keck iodine cell which contains thousands of well calibrated iodine lines. Using these iodine exposures to calibrate the normal Th-Ar Keck data pipeline output we found absolute wavelength offsets of 500 m/s to 1000 m/s with drifts of more than 500 m/s over a single night, and drifts of nearly 2000 m/s over several nights. These offsets correspond to an absolute redshift of uncertainty of about Delta z=10^{-5} (Delta lambda= 0.02 Ang), with daily drifts of around Delta z=5x10^{-6} (Delta lambda =0.01 Ang), and multiday drifts of nearly Delta z=2x10^{-5} (0.04 Ang). The causes of the wavelength offsets are not known, but since claimed shifts in the fine structure constant would result in velocity shifts of less than 100 m/s, this level of systematic uncertainty makes may make it difficult to use Keck HIRES data to constrain the change in the fine structure constant. Using our calibrated data, we applied both our own fitting software and standard fitting software to measure (Delta alpha)/alpha, but discovered that we could obtain results ranging from significant detection of either sign, to strong null limits, depending upon which sets of lines and which fitting method was used. We thus speculate that the discrepant results on (Delta alpha)/alpha reported in the literature may be due to random fluctuations coming from under-estimated systematic errors in wavelength calibration and fitting procedure. Comment: 19 pages, 10 figures, to appear in ApJ, Dec 2009 Many corrections and changes made, some new figures
    The Astrophysical Journal 04/2009; 708(1). DOI:10.1088/0004-637X/708/1/158 · 5.99 Impact Factor
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    ABSTRACT: The MACHO collaboration is carrying out an extensive survey to detect gravitational microlensing. We have recently demonstrated the ability to detect candidate microlensing events in real time, often well before the peak amplification of the event. This has made possible the acquisition of spectra over the course of an event, the invariance of which has lent strong support to the microlensing interpretation. This paper reports on photometric data that were acquired from two sites, Cerro Tololo Inter-American Observatory and Mount Stromlo, in response to the real time detection of a microlensing event. The superior photometry obtained from Chile and the complementary time coverage demonstrate the viability of mounting an aggressive campaign of microlensing follow-up observations.
    The Astrophysical Journal 01/2009; 463(2):L67. DOI:10.1086/310057 · 5.99 Impact Factor
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    ABSTRACT: The recent suggestion that the microlensing events observed toward the Large Magellanic Cloud are due to an intervening Sgr-like dwarf galaxy is examined. A search for foreground RR Lyrae in the MACHO photometry database yields 20 stars whose distance distribution follow the expected halo density profile. Cepheid and red giant branch clump stars in the MACHO database are consistent with membership in the LMC. There is also no evidence in the literature for a distinct kinematic population, for intervening gas, or for the turnoff of such a hypothetical galaxy. We conclude that if the lenses are in a foreground galaxy, it must be a particularly dark galaxy.
    The Astrophysical Journal 01/2009; 490(1):L59. DOI:10.1086/311016 · 5.99 Impact Factor
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    ABSTRACT: The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (~1.1) from lensing by known stellar populations. The timescales ( t ) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2 < ##IMG## [http://ej.iop.org/icons/Entities/hatt.gif] hat t < 200 days to be τ 200 2 =2.9 + 1.4 −0.9 ×10 −7 based upon our eight event sample. This exceeds the optical depth, τ backgnd = 0.5 × 10 -7 , expected from known stars, and the difference is to be compared with the optical depth predicted for a "standard" halo composed entirely of MACHOs: τ halo = 4.7 × 10 -7 . To compare with Galactic halo models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo "background"). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of 2.0 + 1.2 −0.7 ×10 11 M ☉ , which is about half of the "standard halo" value. We also find a most probable MACHO mass of 0.5 + 0.3 −0.2 M ☉ , although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10 -4 M ☉ to 0.03 M ☉ contribute ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] lesssim 20% of the "standard" dark halo.
    The Astrophysical Journal 01/2009; 486(2):697. DOI:10.1086/304535 · 5.99 Impact Factor
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    ABSTRACT: Mean colors and magnitudes of RR Lyrae stars in 24 fields toward the Galactic bulge from the MACHO database are presented. Accurate mean reddenings are computed for these fields on the basis of the mean colors. The distribution along the line of sight of the RR Lyrae population is examined on the basis of the mean magnitudes, and it is shown that the bulk of the RR Lyrae population is not barred. Only the RR Lyrae stars in the inner fields closer to the Galactic center (l < 4°, b > -4°) show evidence for a bar. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies. Given the different spatial distribution, the RR Lyrae stars and the clump giants trace two different populations. The RR Lyrae variables would represent the inner extension of the Galactic halo in these fields.
    The Astrophysical Journal 01/2009; 492(1):190. DOI:10.1086/305017 · 5.99 Impact Factor
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    Kim Griest · Neda Safizadeh
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    ABSTRACT: Hundreds of gravitational microlensing events have now been detected toward the Galactic bulge, with many more to come. The detection of fine structure in these events has been theorized as an excellent way to discover extrasolar planetary systems along the line of sight to the Galactic center. We show that by focusing on high-magnification events, the probability of detecting planets of Jupiter mass or greater in the lensing zone [(0.6-1.6)RE] is nearly 100%, with the probability remaining high down to Saturn masses and substantial even at 10 Earth masses. This high probability allows a nearly definitive statement to be made about the existence of lensing-zone planets in each such system that undergoes high magnification. One might expect light-curve deviations caused by the source passing near the small primary-lens caustic to be small because of the large distance of the perturbing planet, but this effect is overcome by the high magnification. High-magnification events are relatively rare (e.g., ~1/20 of events have peak magnifications greater than 20), but they occur regularly, and the peak can be predicted in advance, allowing extrasolar planet detection with a relatively small use of resources over a relatively small amount of time.
    The Astrophysical Journal 01/2009; 500(1):37. DOI:10.1086/305729 · 5.99 Impact Factor
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    ABSTRACT: The EROS and MACHO collaborations have each published upper limits on the amount of planetary-mass dark matter in the Galactic halo obtained from gravitational microlensing searches. In this Letter, the two limits are combined to give a much stronger constraint on the abundance of low-mass MACHOs. Specifically, objects with masses 10−7 Mm10−3 M make up less than 25% of the halo dark matter for most models considered, and less than 10% of a standard spherical halo is made of MACHOs in the 3.5×10−7 M<m<4.5×10−5 M mass range.
    The Astrophysical Journal 01/2009; 499(1):L9. DOI:10.1086/311355 · 5.99 Impact Factor
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    ABSTRACT: MACHO Project photometry of 45 LMC first and second overtone (FO and SO, respectively) beat Cepheids that pulsate in the FO and SO has been analyzed to determine the light-curve characteristics for the SO mode of Cepheid pulsation. We predict that singly periodic SO Cepheids will have nearly sinusoidal light curves; that we will only be able to discern SO Cepheids from fundamental (F) and FO Cepheids for P1.4 days; and that the SO distribution will overlap the short-period edge of the LMC FO Cepheid period-luminosity relation (when both are plotted as a function of photometric period). We also report the discovery of one SO Cepheid candidate, MACHO*05:03:39.6-70:04:32, with a photometric period of 0.775961±0.000019 days and an instrumental amplitude of 0.047±0.009 mag in V.
    The Astrophysical Journal 01/2009; 511(1):185. DOI:10.1086/306638 · 5.99 Impact Factor
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    ABSTRACT: We present photometric observations and analysis of the second microlensing event detected toward the Small Magellanic Cloud (SMC), MACHO Alert 98-SMC-1. This event was detected early enough to allow intensive observation of the light curve. These observations revealed 98-SMC-1 to be the first caustic crossing binary microlensing event toward the Magellanic Clouds to be discovered in progress. Frequent coverage of the evolving light curve allowed an accurate prediction for the date of the source crossing out of the lens caustic structure. The caustic crossing temporal width, along with the angular size of the source star, measures the proper motion of the lens with respect to the source and thus allows an estimate of the location of the lens. Lenses located in the Galactic halo would have a velocity projected to the SMC of ~1500 kms-1, while an SMC lens would typically have ~60 kms-1. The event light curve allows us to obtain a unique fit to the parameters of the binary lens and to estimate the proper motion of the lensing system. We have performed a joint fit to the MACHO/GMAN data presented here, including recent EROS data of this event from Afonso and collaborators. These joint data are sufficient to constrain the time t* for the lens to move an angle equal to the source angular radius: t*=0.116±0.010 days. We estimate a radius for the lensed source of R*=1.1 ± 0.1 R☉ from its unblended color and magnitude. This yields a projected velocity of = 76 ± 10 kms-1. Only 0.12% of halo lenses would be expected to have a value at least as small as this, while 38% of SMC lenses would be expected to have as large as this. This implies that the lensing system is more likely to reside in the SMC than in the Galactic halo. Similar observations of future Magellanic Cloud microlensing events will help to determine the contribution of MACHOS to the Galaxy's dark halo.
    The Astrophysical Journal 01/2009; 518(1):44. DOI:10.1086/307265 · 5.99 Impact Factor
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    ABSTRACT: More than 1300 variables classified provisionally as first-overtone RR Lyrae pulsators in the MACHO variable-star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of nine types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental and first-overtone) variables, this study increased the number of known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars showed strong period changes. The size, and in general also the patterns of the period changes, exclude a simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type, there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type, the number of remnant components is two; they are also closely spaced, and are symmetric in their frequency spacing relative to the central component. This latter type of variables are associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (≈20%) among the fundamental-mode variables versus the low occurrence rate of their first-overtone counterparts makes it more difficult to explain the Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars.
    The Astrophysical Journal 12/2008; 542(1):257. DOI:10.1086/309530 · 5.99 Impact Factor
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    ABSTRACT: We report on our search for microlensing toward the Large Magellanic Cloud (LMC). Analysis of 5.7 yr of photometry on 11.9 million stars in the LMC reveals 13-17 microlensing events. A detailed treatment of our detection efficiency shows that this is significantly more than the ~2-4 events expected from lensing by known stellar populations. The timescales () of the events range from 34 to 230 days. We estimate the microlensing optical depth toward the LMC from events with 2 < < 400 days to be τ = 1.2 × 10-7, with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum-likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8%-50%. A 100% MACHO halo is ruled out at the 95% confidence level for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between 0.15 and 0.9 M☉, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9 × 1010 M☉, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of 2. This is mostly due to Poisson noise, because with 3.4 times more exposure and increased sensitivity to long-timescale events, we did not find the expected factor of ~4 more events. In addition to a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing. We note that an important source of background are supernovae (SNe) in galaxies behind the LMC.
    The Astrophysical Journal 12/2008; 542(1):281. DOI:10.1086/309512 · 5.99 Impact Factor

Publication Stats

7k Citations
713.61 Total Impact Points


  • 1992–2013
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States
  • 1990–2009
    • University of California, Berkeley
      • • Department of Physics
      • • Department of Astronomy
      Berkeley, California, United States
  • 1995–2008
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 2004
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 2003
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
  • 2001
    • Pontifical Catholic University of Chile
      CiudadSantiago, Santiago Metropolitan, Chile
    • Academia Sinica
      T’ai-pei, Taipei, Taiwan
  • 1999–2001
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada
    • The University of Sheffield
      • Department of Physics and Astronomy
      Sheffield, England, United Kingdom
    • University of Berkley
      Berkley, Michigan, United States
  • 1986–2000
    • University of California, Santa Cruz
      • • Department of Astronomy and Astrophysics
      • • Institute for Particle Physics
      • • Department of Physics
      Santa Cruz, California, United States
  • 1997
    • The Ohio State University
      Columbus, Ohio, United States
  • 1994–1997
    • University of Washington Seattle
      • • Department of Physics
      • • Department of Astronomy
      Seattle, Washington, United States
    • University of Oxford
      • Faculty of Philosophy
      Oxford, England, United Kingdom
  • 1988–1989
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States
    • NASA
      Вашингтон, West Virginia, United States